I guess I fundamentally don't see the problem if one engine was in 3rd and the other was in 4th, so long as they had completely independent controls.
Imagine towing a big trailer down a steep hill. That can apply hundreds of HP to the transmission, yet doesn't hurt anything. Each engine would, worst case, see it like a trailer down a hill helping it accelerate.
Now if the engine cranks were hooked together forcing them to spin the same RPM, then that would be bad if they were in different gears. But that doesn't appear to be the case here, since they can start and idle one with the other off.
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Like I said I could be missing something, but I think in "theory" they are crank to crank (via gears, if you take out the fact that a transmission can act as a hydraulic coupling), and would be an issue if one was in 3rd and the other in 4th, or both in the same gear but one locked the converter and one didn't (again why one TCM makes sense).
Yes one will try to speed up the other (or slow down) and they will fight each other but unlike towing a trailer on a steep hill, there is actually torque being applied vs potential energy. With mismatched speeds either one/both transmission will slip or parts will break. I guess a third option is one engine will stall and the one running will force the other to turn (which is more like the towing scenario where power is only transmitted one way).
In one of the videos he posted (
Source: https://www.youtube.com/watch?v=9wiaZbvAaO0
), just past the 3 minute mark they show a HUGE custom dropbox that connects both engine/trans combos, and transfers the power down to the driveshafts (most likely with gear reduction as well).
While not physically crank to crank, when the transmission are in gear and converters locked, the cranks are forced to spin EXACTLY the same speed or break parts. Maybe there's some sort of clutch or torque limiting coupling to prevent damage but I did not see anything like that in his build videos.
He states one engine/trans is mounted forward and the other is mounted backwards, so I'm willing to bet each input is offset either left to right or up to down with identical tooth count input gears that mesh (because each gear will be turning the opposite direction) and then those drive a gear that powers the HUGE chain down to the driveshaft outputs.
In his truck it seems as if when the transmissions are in gear (and especially when converters are locked), they essentially are connected. What's nice is when the TCM commands Park or Neutral, the engines can be started/stopped independently without worrying about one engine trying to affect the other (beats having to start both at the same time), as the transmission becomes a disconnected hydraulic coupling.
Again, I am NO expert and have not put my hands on this truck personally to see what has or hasn't been done, but I've followed the build for quite a while now as I was interested in how it was done and the dual transmission control was one of the aspects I've been most curious about as to how they were going to overcome the issues associated with that.