Thats fine imo, top priority should be to get the stuff off the healer, they can't heal anyone if they have to heal themselves.
Splitting damage is perfectly fine,we do get aoe heals anyhow, so why not?
As a healer, I'm glad to have people who do this, so I do the same when I'm dpsing and get a lazy/bad tank.
I would likely have thanked you, and not the tank.