I think BigJim is the only way it could possibly work. On each tank you would have to remove the frame, strip out the silicone sealant on both edges of one of the panes, so that you were left with 2 tanks that were each missing 1 of their 4 sides. Then build some sort of stand that could hold the two of them once they were together as one, put them up against each other, and use aquarium (silicone) sealant to seal the two together. However, you would basically never be able to move the tank (I don't think) once they were sealed together, so you would have to do the sealing wherever it is you plan on keeping the tanks permanently.
Or, you could sell both of your current tanks on CraigsList and then use the money to buy yourself one new tank that is large enough to house everything you want to have.
Which might be the easiest option here.