My brother plays a druid not dual box just plain single mode.

His instant cast heals are keybound as follows:
Mousewheel scroll up: Renew
Mousewheel scroll down: Lifebloom
Mousewheel push in: Swiftmend

I'm assuming you're fighting with your warrior main, and the healer as side kick on follow. You're going to be limited to the instant cast heals above unless you pause to heal (which i dont recommend unless you're snared/rooted only). Try those mouse bindings out, I'd imagine they aren't bound to do anything for your warrior. That way you can multitask in your head easier. Keyboard=warrior / mousewheel=druid heals. I really don't know much as I'm a beginner myself, but hope it helps.

Oh get used to dieing as a warrior in BGs that's what they do best. :S

I've been playing warrior/hunter, warrior/warlock, warrior/shaman but performance wise the companions aren't performing at optimal levels (yet, I'm still learning).

From what i'm gathering so far in my own experience, contrary to the opinion warriors need heals to perform well, for a dual box setup, it may be more efficient to run a dps sidekick instead of a healer. The burst damage of warriors is already amazing, why not pair more damage with damage?