ORIGINAL: I-Love-Jets
For instance, if you look at the German Speed Cup contenders, nobody will ever use a twin-power setup. Even the inline tractor/pusher configuration is not worth a try in case of serious nitro powered speed attempts.
I think in theory that this might be about the only arrangement that might ever offer a speed advantage by using two of a particular engine rather than one - since it seems to me you may be able to double the power without increasing the frontal area much or at all - if you line the pipes up anyhow. I doubt it offers any benefit in practice, but..?
MJD