I've seen this before and I'd say it's pretty safe. You should see the ones made out of acrylic, they're amazing.
The filter part or the connecting?
Well anyway, since the two bodies of water are now connected it's like keeping the fish from the 2 tanks in 1 tank that's the size of the two tanks combined so all of the pollutions will be diluted.
I believe that if you do a water change on one it's like doing a water change on both.
tlindsey got any experience here?