One Engine, One Transmission, One Differential, Twice The Gears
 

This idea has morphed so far from the two engines, two transmissions string, I thought it deserved it's own string:

Aerodynamic one person reverse trike.
One Motorcycle engine, with it's five speed transmission, Two output sprockets, each feeding one side of a Spur Gear Differential, with a dogged plate on the front sprocket of one side able to engage or disengage the front sprocket on that side, and a disc brake caliper on the rear sprocket of that side to hold that side shaft of the differential from turning when the front sprocket is disengaged. Differential feeds rear wheel by sprocket on the carrier case.

Speed in ratios would be:

Left/Right Feeds-------Final Ratio--Top Speed
1/1-----------------------13.990 --- 47 mph
2/2 ------------------------9.540 --- 69 mph
3/3 ------------------------7.450 --- 88 mph
1/Disengaged, braked ---6.995 --- 94 mph (Essentially halves the gear ratio)
4/4 ------------------------6.140 -- 107 mph
5/5 ------------------------5.260 -- 125 mph
2/Disengaged, braked ---4.770 -- 138 mph
3/Disengaged, braked ---3.725 -- 176 mph
4/Disengaged, braked ---3.070 -- 214 mph
5/Disengaged, braked ---2.630 -- 250 mph

I have not taken into consideration Differential Ratio, but that can be handled
by choosing proper larger front/smaller rear sprocket pairs.
Here, the ratios compared to just the five speeds:

Comparing CB750 + Differential to standard CB750 gears alone

2 in/1 out----Standard CB750
47 mph-------------47 mph
69 mph-------------69 mph
88 mph-------------88 mph
94 mph
107 mph-----------107 mph
125 mph-----------125 mph
138 mph
176 mph
214 mph (probably won't go this fast, but the gears are there.
250 mph

Forgot to say it, but, shifting the transmission would be done by a paddle shifter actuating an up-pull servo in one direction, for upshifts, and a down-pull servo in the other for down shifts.

Differential locking and one side de-clutching would be by a rocker switch on the other side of the steering wheel.

The goal is what?

CB900 Reverse Trike is heavier than CB900 Bike, and needs the extra gear between 3rd and 4th for acceleration. Locking one side of the differential gives 4 higher gears for cruising. May not be possible to use all of the higher gears anywhere but downhill. (Might pull the whole range downward so that acceleration is quicker, and cruising gears are more usable) A better diagram of the powertrain:

If you are using a differential backwards then locking one side is going to make your gears half as tall, not twice as tall.

I think you are right! Will have to re-think.

OK,
Re-thought.
I'm wrong!