Do weapons that have lifesteal and hit multiple enemies per attack heal you for each enemy hit?
I am pretty sure it is just for the primary target. I have tested this by setting Damage text setting to verbose and got myself a cannon on an engineer with +8 Health per Hit. Even though my cannon was hitting many mobs I get only one +8HP sign. I double checked my health globe too. It says the same thing. IT goes up by 8 every time.
So the short answer is you get only 1 +Health per shot/hit.
Otherwise imagine the cannon engineer exploiting a 4 slotted cannon with Blood Embers and blasting away at crowds... virtually invincible!!