Here is a
recent attempt to do a full sized 2 way system.
I don't think the few 2 (or even 3-way) full sized headphones sound(ed) any better than single driver.
Also planar stats and planar dynamics have peaky treble and the headphones with small dynamic drivers also are peaky in the treble.
I once measured an excellent tweeter from just 2cm away and it too measured peaky.
I suppose it has to do with shorter wavelengths interfering or break-up or local resonances being measured.
A headphone would be equivalent to an almost unison moving wall from a few meters away funneled in ones ear ?
A headphone just is what it is (an alternative to speakers under certain circumstances) and one has to accept its shortcomings.
Having heard and measured a lot of great and not so great headphones I am still amazed how good headphones can sound with well made recordings.
I don't see the need for 2 way systems here at all. I don't think it will solve anything.
One would have to make a really small (almost point source) tweeter that would then need to have quite an excursion which won't help fidelity.