If you time everything to happen in the lead engine just a couple of degrees ahead of the other (if coupled at the crank), each power plant can act on it's own and the lead engine will always tend to keep the coupling tight, even if it makes far less power but can keep up with the revs.
A crank coupling with no ability to have just a little flex will cause a lot of problems to be transmitted to the other.
If the fat guy and the skinny guy on the scale doesn't compute, how about the big guy and the little guy both pushing a car for all they are worth ?
It works for girls too.
Think little power and big power.
Q: Who is gonna get there first ?
A: The car will get there first and won't care who did how much.