I just tested out Sylvan embers with a Druid friend of mine and the spell stacks on mobs between Druids.

HOWEVER--- does anyone know if the debuff portion of that dot stacks?

it shouldn't, I know embers overwrote immolation of ro b/c of the debuff portion. Best bet might be to try having the 2 of you put it on a tank in the areana and have them watch their fr when the 2nd one is to land.

You can't stack buffs on players.