Biamp N802:is 55W tube enough for high/mids?

As Arthur notes, you should be able to do it. OTOH, 350Hz places you smack bang in the middle of the voices range, so most of yr tenors will come out of the "bass" amp and most of yr sopranos fm the triode (likewise with cellos and violins):)

Just check that your triodes of choice sort of maintains constant power between ~3,5ohm and ~8-10ohm.

Of course, you will test this before you buy, right?

Anyway, depending on how the triode behaves under test, you can guess at the end sonic result from an amplitude point of view.

The key is that I can actively biamp and eliminate any overlap between amps. I will be using a DEQX that can do 100db/octave+ crossover

I strongly suggest you follow the characteristics of the passive x-overs (slope, freq, q, etc)

You will disconnect the spkrs' internal x-over of course.