Im not sure, I wanted to test this out. Use those 2, deep wound, and Malign Intervention. You could also be a n/me for heavy dot and Phantom Pain for deep wound. Also packing Soul Barbs might help out for increased damage.
I suppose it does stack... it is like the speed buff... it does not go over one skill allow it (the more effective one take place)
so basically the max is 77% healing from 1 skill, and would still be 77% if it were 2 skills... however, if something that increase healing % before hand... so you start from 120% - 33% - 25% = 77% the 25% merely make up for the 20% from earlier.