I do like the idea of HP sharing but being the damage tank in this game is difficult. Monsters will often ignore the players who have done little damage meaning they won't target you if you sit in the middle just shielding and absorbing hits.

I could see this work if OOO start adding new abilities to armor sets. This armor set would require you to fight to fill it up, when not fighting it would slowly go down. Once full however it stays full till activated and when you do it heals a percentage of your ally's HP (provided they are alive).
I would certainly enjoy using this set since I tend to use tankier sets anyway.