Having 2 driving wheels on a common axle calls for a differential so that one can go faster (or slower) when going around curves. These are pricy and heavy so cheap trikes opt out and have single wheel drive.

Years ago there was a trike (don't ask, I don't remember) that had both wheels share a common axle, but each wheel was on a freewheel and so the outer wheel on a curve could overrun the inner. This is a nice simple and efficient design, except that it can' be used with any kind of hub brake, which is why you don't see it often.