I have a sump made up of 2 50-ish-gallon plastic tubs, just hooked up in series. Tank overflows into one end of #1, overflows at the other end into tub #2 through 2-inch pipe, and then pumps back up into the aquarium. It would be very easy to do with a pair of glass aquariums, since you wouldn't need to worry about the side walls flexing as the plastic does. My tubs are side by side, with short pieces of pipe connecting a drilled hole in each one. You could do the same, or if you have enough room, you could put the first tank on a support that holds it a few inches higher than the second. Drilling the first tank near the top would then allow it to overflow directly into the second tank set below it.
As long as you can drill your tanks accurately and neatly, this would be way easier and more trouble-free than trying to actually attach two tanks together. In fact, you could series-attach all three this way and probably wouldn't even need to think about more partitions since you will already have 3 discreet compartments. I think that about 90% of the partitions seen in sumps are used because the owners think they look high-tech; in reality you need few or none. IMHO the majority of partitions tend to just get in the way and waste space.
Sumps only 12-inches deep are a bit limiting, since they overflow quickly as the aquarium drains down to the lower overflow limit if the pump goes down. If you do the 3-tank thing, the last tank could be entirely free of partitions and would give you a decent capacity for catching this overflow water until the pump turns back on.
