How are you presently using the "pair" of MOSFETS? Synchronous buck converter or are they in parallel?
If it's designed around 100W, I wouldn't be surprised if it starts to get hot at 300W but would be useful to see a schematic if you want a more precise critique, the waveforms don't look too bad.
For the circa 30V @ 20A mark, I would start to consider an interleaved design. It would be possible to just design a single buck stage at that power rating, but you can probably optimise an interleaved pair better. It is also preferable with PV inputs to keep the input ripple current low, another thing that an interleaved solution would allow, (considering the discontinuous input current of a buck (I'm assuming it's a buck you've built, please correct if wrong)). If it were a normal stiff voltage source, I would suggest 2 interleaved is probably enough, but my gut instinct would be for 3 with a PV source at those ratings.