The current wording for Healing Presence is: "Attach to target. At the start of each round, heal 2 to all adjacent allies" I think the wording should mention if it heals the caster's allies or the target's allies. I haven't tested it myself, and i don't actually know the answer, so i think it's a good idea.
I'm 95% sure its the casters allies based on what i think i remember from using it a long time ago. Should be super easy to test tho just chuck it on a enemy and hug it.
All it takes to make it clear is target's allies or caster's allies, 1 extra word. And it can come up, if for instance you will be fighting an enemy in melee, or you would rather have both allies at the position healed rather than one. Also it can be used to push off buffs without giving them an advantage then.