I'd love, at some distant future occasion, to try and build a sahara-like double engined car. This will not be any time soon, but I devote a little brain power to it now and again and inevitably questions crop up. The one that hit me last night was:

From my understanding, cars such as these can be driven on either, or both engines. Is there some sort of mechanism in place to disengage the clutch of an engine when its not in use? Surely if you were driving on, say, the front engine and 'towing' the rear, you'd not only be fighting the compression of the rear engine, but also the fuel pump etc would still be running off of the spinning camshaft?

Or have I completely misunderstood the mechanics of these cars in some major way?

The way I saw it on a racing Mini with 2 engines was that he had installed a joint in the linkage to the back engine so the gear shift could be disconnected to that engine when in neutral and then he drove on front engine only. When goin back to 2 engines stopped, into neutral, engaged link and away with both engines/gearboxes working. Maybe different with 2cv...

That would make sense, and like you say it'd let you run either front only or both engines at once.

Still curious if there's a way to disengage the FRONT and run just the rear engne.

Yep some sort of disconnection in the gear linkage so that one engine can be left in neutral. The 'fun' part is trying to design the linkage in your head so that selects the same gear in both 'boxes, cables are cheating!

It would probably be fine to have both clutches permanently connected to the pedal, especially as the 2cv clutch is so light. You might want to disconnect the accelerator cable from the non-running engine though, to prevent it being flooded by the accelerator pump.

Another thought I had was whether it would be useful to have some sort of rev limiter, that prevented the revs of one engine exceeding say 110% of the other. Might be useful if one end loses all traction when off road.

Yes I've thought about this before.

Good man! I can come to you for someone to bounce ideas off of, then

I love designing things in my head. I tend to keep a notebook nearby too, so i can jot things down if they come to me at random times, like when i'm in the bath

I wish I was as organised as a notebook. I tend to walk around with pockets full of of old envelopes with lists on them. Any ideas worth keeping go in a particular drawer, but there aren't many!

original sahara had the 3 selector rods in the gearboxes linked together by having really long ones!

best ive seen was a gear lever like a normal car imagine a ball pivot with a stick. Front gear box linked to the stick above the pivot ball and the rear connected below the ball when the gear lever moved to right then the front selector moves right but the rear one connected to the bottom moves left ....simples

Don't follow that exactly, if the gear stick moves right the linkage to the front is rotated clockwise, towards 4th gear. When looking backwards towards the rear engine the linkage is rotated anticlockwise, towards 1st/rev gear.
It's easy enough with multiple linkage rods and bellcranks, the hard bit is keeping joints to a minimum and using just one rod to each box.

One neat trick the Mini man had was the rear engine throttle cable going past him so if he wanted to get the back engine to rev harder (to over steer) he could yank the cable......

ill draw a picture later