With my team I have my shaman help heal through the rhino phase on the last Gundrak boss. Otherwise, I've always been a solo, holy priest healer. Might be worthwhile to learn encounters but I think once you a) get geared a bit and b) refine tactics all the multi-box "doable" heroics don't optimize with a 2 healer setup. Probably obvious.

Arenas? No clue.