If you are planning t fuse tanks together by removing the sides from both tanks so they literally make one tank.....then that is a bad idea. Its basic math, you can mold two ten gallons together, or buy a twenty gallon, either way its the same amount of water,, but one way is right and the other very synonymous with wrong. The smartest and coolest way to do this, if at all, would be to cut two holes in the corresponding sides of each tank, run clear acrylic tubing or pvc to couple the tanks, the length of the tubing will depend on how far from eachother you wish the tanks to be. Hot glue is no good and super glue isn't so super. If you still wish to embark on this project i would use a silicone sealant, or pvc cement around the outside of the tubing and use JB Weld around the inner rim of the cut glass to bond the glass to the tubing. But the best idea is to disregard all of what i have just said and pretend the build is impossible.