Even with the forced duo and activation cost, this seems like it may be way OP compared to empathic bond.
Its an interesting concept, but I think a direct linear scaling may not be the way to go... Maybe something along the lines of catapult scaling, but weighted to the 20ish range instead of 100.
(HP*20)/(HP+20) where HP is the sum of the player's creature's HP values.
The 20 is just an example, it can be changed to whatever seems like a good cap should be... It could even be made to depend on stack size if you want this to be stacakable... maybe 15 +5 per card in stack?