Re-keying-- single/double cylinder

Yes you can use the same key. All other things being equal and sometimes even when not.

I was hoping to have

If the double cylinder lock takes the same blank i.e. has the same keyway as the other three then there should be no problem.

Each of the cylinders SHOULD be of the same manufacturer.

All Schlage

All Kwikset

All National, etc.

If you have a number of different lock manufacturers, then, you'll have to change some to match others.

Newnsie

there comes in a life safety issue with a double cylinder lock, just so you know.

--Shiva--

The inside cylinder could be masterkeyed and the 'servant' key left in the cylinder when the house is occupied.

Yes, it is. If the locks are the same brand and keyway.

Still possible in some case, mixing brands. Today I went on a call where I had Kwikset door knob, Kwikset deadbolt, Titan door knob lock, Weslock door knob lock, and Weslock deadbolts. I used Titan keys, and keyed them all alike.

Ideally so, but not required. Some lock brands can be cross keyed to each other.

I've done that. I usually cut it on a different blank than the owner's key. For example, if it's a Kwikset system, I'll use a Dexter blank.

And I've called it an "escape key". Lets you escape if the house is on fire.

titans are -6- pin, the others are not

Thanks to all for your responses! I really appreciate it.

Yeah, but Titan uses a key with two different shoulder heights so that it can be used in a 5 pin lock. Only the 5 cuts nearest the tip of the Titan key are used in a Kwikset.

Daniel