Looked into this myself a few times as ive got a huge interest regarding twin engined machines.

A feature a while ago about the twin engined mk1 golf addressed the problem which you spoke of regarding one engine 'carrying' the other. The owner stated that it's a bit like 2 blokes pushing a car, 1 strong bloke, 1 smaller bloke. Although one bloke is doing more work than the other its still better than just the one

Will DEFO be building a car like this when i finish my Zetec Turbo project properly.