Pawel
it's not such a good idea to have the rear axles so wide, even if you only consider the torque they have to transmit. In such cases you usually aim to have the half-axles as short as possible to handle the torque better.
I have to agree with Pawel here.
So, I'm thinking about how you'd get the power out of the engines. Easy guess would be a pair of torque converters into a common shaft to a transmission. From the transmission out to a transfer case which would have four outputs to half-axles to the rear wheels.
The cab would have dozer-style controlls to throttle the two engines, indexed back to a single accelerator pedal (or, perhaps, a single deccelerator pedal). Note, I'm trying to concieve of a way to present the driver with a simple-enough controls layout. Which is why I'm thinking a foot pedal not for a clutch, but a motorcycle style shifter pedal for the transfer case.
But, this build retains a huge "cool" factor. It's Wastelan Chic before the Wasteland.
The individual rear fenders are for sure the way to go. Tha long chrome box between front & rear fender would also be the way to go, too. (Mind, I just had a twisted thought about how big the the DEF tank and DPF tank would want to be to meet CARB . . . )
One of those fender boxes would need room for the pony engine to drive the air compressor, too--no reason to rob power (esp. after gearing down) to drive compressors off the main mounts. A Wnakle diesel would be compact enough, and give a nice reservoir for starting the beast, too.