Really like this concept - very cool show tank.
Another workaround would be to very simply use the power of the Siphon! http://wetpetssupply.com/images/utubeCG89194medup.jpg
- just a tube like this hooked over the middle of the two tanks. Use an airline to suck the air out and when the water level is higher on one side than the other the siphon will equalize the pressure by flowing water into the tank with the lower level. A 3/4 inch ID tube could probably do more than 600gph.
To prevent migration sponges would have to be used on either end.