OK: The design has moved forward from what I originally posted, partly because of the feedback I've received, here.
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You obviously have dealt with such things only on paper.
Yes, this is true, so far.
Your one engine at 90 degrees will put the whole length into an unbalanced condition with different rocking moments on both halves of the overall 'engine' thus breaking parts.
I have recently decided to put a cush drive between the two crank shafts, to soak up such problems .
You have not given any thought to trans shifting as in how? Sometimes you will shift one engine and sometimes two, how to memorize when and how?
Yes, I would have paddle shifters on the steering wheel, one on the right for the right hand engine, one on the left for the left hand engine. Each would operate an electric servo on it's own shift lever. Each paddle shifter would also operate an electric servo on it's clutch cable, and ignition coil cutoffs on both engines, so engines would not run away during shifts, and the accelerator pedal would not need to be lifted during shifts A separate clutch pedal, operating both clutches, would bypass the cutoffs for pull away from stops. I would mount the bikes' gear position lights on the dash, to remind me which transmission needs to be up shifted next.
We've already looked at what will happen when you let one side of your differential go unpowered for a second.
The aforementioned dual engine ignition cutoff switches during shifts.
Don't remember if we went over it but if the trike weight overwhelms the one motor not at power then the high power setting motor will defer to trike weight and use the diff case to try to spin the low power sprocket BACKWARD thus tearing engine up.
I have relocated the spider gears to within the rear wheel hub. , driving them from the two large (sun) gears, one on each side of the hub, (search 1901 Locomobile Differential for the layout, though mine would drive from the sun gears) When both sun gears are rotating at the same speed (same gear) the spider gears would be locked,(pulled forward at the same speed on both sides) and the wheel hub would rotate forward at the RPM of the two sun gears. When one sun gear is rotating faster than the other (different gears) the spider gears would rotate on the spider , somewhat like rolling a pencil between your two palms, and the spider and wheel hub would rotate forward at an RPM that averages the two sun gear RPMs, thus giving a gear ratio between the two inputs. With the two crankshafts joined, There would never be a case where one side is held stationary.If one engine "dies", it would still rotate at the same RPM, being pulled through the cush drive linkage by the running engine.