You will likely add only a tiny bit to the top end. Max RPM is max RPM.
But it will pull much better .
Witch will allow for a smaller tooth count sprocket . And ultimately a better top speed....
If you link the engines from the output sprockets , you will only need to control the down hill clutch.
The best information on this subject can be found on any karting sight.
We have been running twin 2 strokes in the Go kart world for years..... Most are vintage karts...
I like the idea though.
I might try it on a 3 wheeler running centrifugal clutches...... ( no need for engine timing as the clutches will
compensate for the engine differences ) .