I drew this crudely but the combination design could possibly do the best for what you're wanting to do. It would be the same as the bridge design only you'll be attaching them together with out the bridge(oviously).
One important thing to note is if you do combine them like that then you'll will want to redo the rim around both bottom and top to retain the rigidity. Also you'll want to seal where the two tanks meet in the middle by applying a small beade of silicon between both tanks and also applying a smoothed beade on the edges once the are joined.
This is of course assuming they're square to begin with. If they aren't square I would go with the acrylic bridge design.