2 Loops, 2 Controllers.

To begin with, I strongly urge you to connect with other modellers or a club near you, and get some face-to-face advice. Trying to explain this stuff without sketches is dam' difficult. Bring a pad of paper with you, as there will be lots of drawing and sketching! ;-) You should, I think, also invest in a book about electrics and model railways.
As drawn, the crossover and a section of track on each oval must be isolated from the rest of the layout, and selectable by either controller. But you'll also need to select either oval by either controller, so:
First and foremost, always think "controller = train", never "controller = track". IOW, arrange things so that any section of track can be connected to and controlled by any controller (in your case, two.) Use a double throw switch (1) to connect each section to either controller.
Second, operating trains on the layout as drawn. Yes, parking one train in the siding while the other crosses from the inner to the outer oval (loop) will work (ignore the electrics for the moment). However, you will have no way of getting the parked train into the inner oval, and to get the other train back onto the inner oval will require backing up. IOW, the layout as sketched is really a "one engine in steam" line, and would need only one controller. If you put in another crossover between the two ovals, facing the other way, then a train can cross from inner to outer and back again without backing up, and vice versa.
Third, operating two trains on a simple layout: a there should be an isolated passing loop on each oval (two on each oval would be better yet.) Then you can park a train on the main or in the passing loop while the other does its thing. The siding would not be needed, but you can of course have as many sidings as you like for other (operational) reasons.
The electrics for running two trains are rather complicated, even for this simple layout. Read on. ;-)
Fourth, the electrics: As mentioned above, you will need a crossover section that covers _both_ ovals. The scheme described in point 3 (with a passing loop) will require four or five electrical sections on each oval, plus a common one for the crossover: 1A, the passing loop, 2A, the mainline parallel to passing loop, 3AB the section with the crossover, and 4A the rest of the oval (and 5A, depending on where the passing loop and crossover are relative to each other.) A second crossover would require even more electrical sections.
You'll need 1B, 2B, etc on the other oval, too. Sidings such as you have drawn should also be isolated, and thus each would be an additional electrical section.
And we're not finished yet.... ;-)
Whatever operating scheme you devise, on any layout, collisions are possible. You avoid this by A) adding control sections (= ones not required for electrical reasons alone); and B) wiring and switching the sections in such a way that connecting one to controller A will disconnect it from B, and vice versa (use double throw switches and common earth wiring for this, or you'll need more complicated switches and more wire); and C) wiring the layout so that switching a section to A will disconnect both the sections in front and behind the train from B (thus protecting the train in prototypical fashion.) Double pole switches are needed for this to work.
But judicious use of control sections and signal cabins (with signal men to operate them) will prevent collisions. This also means that you may need at least one other person to run trains, which makes for a much more sociable time. ;-)
As you can see, even a simple layout like the one drawn and amended by me requires a lot of switches, wire, and sometimes knotty logic to set up for DC (direct current) operation. That's why DCC was invented. ;-)
And we haven't even touched on wiring up the turnouts (which can be done so that the turnouts control some of the sections).
(1) For switches: Pole = electrical connection. Throw = connection made or unmade. EG, a single pole, single throw switch connects one circuit (this is the on-off kind you turn your lights on and off with.) EG, a double throw switch connects with one or the other of two circuits. It's best for it to be "centre off", ie, the positions are "on-off-on." On many controllers, this is built into the speed control knob.
Have fun! wolf k.

I remember how confused I was when wanted 4 controllers to control a number of lines. Drew lots of curious circuit diagrams ....then realised the basic concept is each track wants one and only one connection at all times, whereas controllers can have one or multiple at any time.
So first isolate the loops with one set of plastic joiners connecting track between the loops. For first loop connect to output of double pole, double throw switch. For 2nd loop connect to output of a different double pole, double throw switch.
Connect first controller output to one input of both double pole double throw switches. Connect 2nd controller output to other input of both double pole double throw (DPDT) switches.
Now as to 2 engines running - assumes you have one controller (A) switched to inner via DPDT switch and one to outer (B) via other DPDT switch.. So to move engine from inner to outer :- Move engine of outer to siding. Set outer DPDT switch from controller B to controller A - now controller A runs both loops. Close points. Engine runs from inner to outer, still under control from A Open points.
You can now continue to use controller A on both loops, OR Set controller B to same speed as controller A. Set outer DPDT switch from controller A back to controller B.
You are back where started, with controller A on inner and controller B on outer. Engine has moved without a hiccup (ok minor one cos controller speeds slighly different).
Does this help ?
Plus if you wanted more controllers but still 2 loops then replace DPDT switches with rotary switch. Still each track has one connection but controllers connected to all switches.
Cheers, Simon

explains the difference between section control (passing trains between different controllers) and cab control (switching the complete route for a train to a single controller).
MBQ

Thanks to all the info...I'm not electrically minded much. I've managed to get round this by simply having a switch in my controller lead which stops any current from going back into it from another controller. I know it sounds primative but it does what I need it to...
-- The Zero ST

If it works for you and you can do everything you want then tis exactly correct.
cheers, Simon

The only problem there is that you need to remember to throw the switch _first_. A very basic safety addition is to solder a motor vehicle tail lamp bulb in series (ie in one lead). About 25 watts is right. With a train running the bulb will only just glow and you'll lose about half a volt. If you forget the switch then the bulb will glow brightly to remind you without any damage to anything.

That's all very well until the bulb burns out. Then you get an open circuit, i.e. nothing works.

[...] Use screw terminals.
wolf k.

Why would it burn out??? I first tried the technique back in the early 1970s when I started to build transistor controllers. I've never had a bulb burn out, even after 20 odd years of use. They only get a tiny voltage heating them. Mine were old dual filiment bulbs with one filiment gone - useless on motorcycles :-)
You could get two controllers on full speed and opposite polarities connected across the bulb - that would blow the 12 volt bulb, but it would burn both controllers out without the bulb. If you're using early H&M point motors you might vibrate the bulb filament until it broke, but your trains would keep falling off the track so you'd know! ;-)
Greg.P.

But they're at least one order of magnitude cheaper to replace than a controller. And fairly unlikely to burn out in this application.
MBQ

Quite so. A single controller should not deliver more than about one ampere for OO use. A 25 watt car lamp will rate at 2 amps, but will handle about twice that before it burns out.
BTW, some older controllers will deliver too much power, which IMO is the other reason they should not be used. Main reason: non-standard grounding/isolation from earth.
HTH Wolf K.