With the introduction of the co-op levels I thought it would open up the possibility to send one of my portals through my partner's already established portals. This would allow me to create a portal in a place that might not be accessible otherwise.
I think the game is capable of supporting this behavior but when I tried it (and I have tried many times during the co-op game) all it ever did was ruin one of my partner's portals.
Are there special test chambers where this is allowed? I have not finished all of them yet.