Ion Shell currently does not stack at all, meaning when 2 different heroes cast Ion Shell on the same unit, only the last cast works. The previous cast gets completely overwritten. This means an enemy Rubick can currently completely negate all your Ion Shells by simply following up with his own Ion Shell.

It would be better if it would stack per player. So if player 1 and player 2 cast Ion Shell on the same unit, the unit should have 2 shells on, one shell damaging enemies of player 1 and the other shell damaging enemies of player 2. When these players cast Ion Shell on that unit again, it should refresh their respective shells.