Honestly the more I think about it the combination probably wouldn't serve too well when considering glass thickness it would be very very fragile. If it wasn't on a solid base just a sag in the floor or stand over time with all of the weight would find a weak spot and come crashing out not to mention something bumping into one of the long panes. As far as the seams go at the joint of the two tanks like I vaguely showed above; you could easily hide with proper scaping and display point of the aquarium.
Glass thickness is key to combining them.
Building a 75g all glass tank
The guy in the link above provided great information in his build journal.
If the glass is too thin then I would consider making a bridge.
A 6x6 trough at the top leaving a 2-4in gap between the tanks would be better but you'll want to try and save the rim around the top. I personally wouldn't try going bigger than that. I'd like to add that if you did the bridge then I would beef up the glass or acrylic(which ever you choice you make) thickness and use the correct silicone definitely do not cheap out on that part.
Like I said before it just comes down to the thickness of the glass.