Did the twin engined Lazair ultralight have a throttle for each engine?
I've examined a Lazair, and actually taxied one around on the ground once, and yes there are two throttles.

Two reasons:

In any twin engine airplane you want to minimize or eliminate single-points-of-failure (especially with engines as unreliable as little 2-stroke water pump motors) and this means you want the keep each power plant completely mechanically independent all the way up to the throttle.

You need to be able to adjust power independently to synchronize motor RPM (because the wowowowowow of out of sync motors will drive you bonkers), as well as control each engine independently for starting etc. 

So, a single throttle to control both engines is a very bad idea.