Hi
Well youre right not to rely on the zooms onboard mics. Theyre really not up to much and have very little reach.
If you use 2 shotguns to sep record tracks, you only run the risk of phase cancellation if you leave both open together and mono them.
Id record the 2 separately to the zoom split track, and then mix them “bulldozing” the faders gently so that only the best, nearest is up and the other is decreased (not off)
Make sure the cables you use for both mics are the SAME length.
Id position each mic just out of shot NEXT to each other but pointing out over (triangle shape) but only if the performers are to be fixed in position.
If they might move out over a little (apart) then go for 2 positions wider apart pointing in over.
Be clear that phase cancellation only happens in mono and when both mics have a fairly equal value in the mix. Avoid this and you will be fine. LISTEN on cans in MONO to check phase id good.
Personally Id want 2 lavs on them also to PIN their position in the mix, but if you havent got lavs and arent going to get them then ?
Peter
Post Production Dubbing Mixer