From a game point of view, the code currently assumes that all engines are linked via a shared transmission or other mechanism, which is the justification for continually increasing installation requirements of additional engines.
My idea for correcting this is to have a separate transmission part that has its own installation requirements, and can be linked to some subset of the installed engines.
Simultaneous operation of multiple engines would still have high requirements, but these would be based on the transmission rather than the engines themselves.
On the other hand, a transmission that allows driving the vehicle from only one of the engines connected to it is much simpler, and the complexity doesn’t keep going up as you add more engines, as long as only one of them can run at a time.
Likewise, you can install an engine and only connect it to one or more alternators, using it just to generate power without impacting the performance of the vehicle otherwise.
tl;dr Add a transmission part, when you install an engine or a transmission you get a popup or something asking which transmission to connect the new engine to, or which engines to connect the new transmission to. Transmissions are implicitly linked to all wheels, a side effect of this is that you can only install one transmission (or one functional transmission?) per vehicle.