Thanks folks.
Model of the Year at the Model Car Club was postponed until next month due to only two of us bringing entries.
Um, there might be a simple answer to this, but with the rear engine facing backwards like that, wouldn't that make the rear axle run as if in reverse 
Transmission for the rear engine spins opposite the front transmission.
No, I thought of that. Initially I was going to say that the rear engine was modified to run backwards (not too hard to do - basically* only a retimed camshaft is needed) but then I realised that as the rear engine would require higher overall gearing then I could kill two birds with one stone by adding an extra gear in the transfer case which both changed the direction of the drive from the rear engine and geared it down.
*OK, so it's not quite that simple but it isn't rocket surgery.
Fantastic!
Would that be street legal, and would it work in real life?
Legality would depend on which engine you were using. If it was the rear one then then answer would be
WHAT DID YOU SAY? I CAN'T HEAR YOU OVER THIS ENGINE...If it was the front engine then surely it would just be like doing around with a spare engine in the back...
As for would it work? I like to think it would. I put a lot of thought into figuring out how to do it so you could run on either engine.