When you superpose two 100w coherent laser beams, the resultant
power is indeed 400w
This is correct if the two beams are coherent and have the
same polarization. Very hard to do at optical frequencies, much
easier at lower frequencies.

As in coherent RF waves confined to a transmission line, eh?

That's nothing new. Why do they call them "waveguides" ? Jackson
has a chapter with all the hairy details.

Tor
N4OGW