Ok I have to ask a question here. How long do you want the tunnel? If you dont mind it short why dont cut the holes in the glass and you could silicon the two tanks together, this will solve the problem of things moving and leaking by having the two tanks formed to make one. The other way is if you wany the tunnel a bit longer say 5 inches, what you will have to do is stop the two tanks from moving. How you would do this is first seal the plexy glass tube into the drilled holes. You have to stop the tank moving by glueing a ridged board the lenght of the two tanks put together plus the length of the tunnel to the base of the two tanks. You will then have to get some thick plexy glass the thicker the better ,the same width of the tank and the same lenght of the tunnel and glue that two the top of the two tanks sort of forming a bridge like thing.This will secure the two tanks together and stop any movment around the tunnel when you move the tanks. If you want me to clarify anything or if you dont understand what i have explained , please tell me.This project is not impossible.