My dad had a brainstorm back in 1952 to build a Semi Tractor that would eat up the road and over the mountains. It had to be light weight and short as for the East coast states had 45' overall length laws, He started out with 2 pieces of frame, used rearend housings from military 6X6 so they were offset to allow the second driveline over the 1st rearend to power the rear rearend. Each engine drives each rearend, they adapted the housings to accommodate 2 speed Eaton Rearends behind the 4 speed Dual range Hydro Matics (they say that's a no no to have a 2 speed rearend with an automatic say nothing about 2 of them But he did it) His concept was not to synchronize the 2 engines But have one engine keep pulling while you shift the other one especially on the mountains so you wouldn't fall back grabbing the next gear, AND IT WORKED, HE NEVER GOT PASSED BY AN OTHER SEMI ON THE MOUNTAINS, AT NIGHT TIME GOING UP LOOKOUT PASS WHEN HE WOULD PASS UNITED BUCHINGHAM SEMI WITH A 280 BUTTA THEY WOULD HAVE A FOOT OF FLAM OUT THE TOP OF THE EXHAUST STACK TRING NOT TO GET PASSED! IT WORKED! He put 2 brand new 302 GMC inline 6cyl. engines with 2 brand new heavy-duty Hydro Matics, each engine driving each rearend, 2 complete wiring systems, 2 side by side gas pedals that 1 foot can control, 2 brake pedals, the one on the left controls the middle axle and the right brake pedal controls the front and rear axles, everything has TWIN gauges The cab was used on different makes like International, Diamond T and others with an internal sleeper cab, It is very SOLID AND RUST FREE, has not been run in many years Hopefully someone will resurrect it and not part it out. I will try to find more info and pictures