How do you control a pair of ducted fans (or dual props) to steer, etc?

It's called a mixer. You can get them as separate circuits that go between your receiver and speed control, or you can get a transmitter that has mixing (make sure it has the kind you want).

You need two channels -- throttle and rudder, probably.

No rudder proposed.

Steering entirely by throttle.

This is a very slow utility barge.

Does this cahnge anything?

No. You want to control two independent things -- speed and differential speed, or speed 1 and speed 2. Either way you need two channels.

If you want the side-side movement of a stick on the transmitter to translate into differential speed, you'll want to use the rudder channel (assuming you have ailerons).