Is the 6 X 3 X 2 taller or wider? Will the two tanks be situated near one another?
If I wanted to coordinate the two tanks I might go with an Amazon theme, with mid-sized cichlids (maybe Geophagus, Crenicichla, and Pterophyllum), silver dollars, and lots of small-to-mid-size catfish in the smaller tank, and big cichlids and cats in the larger tank. Decor and plants would be similar in both.
Actually, if I had those two tanks, I would probably set up the larger one as a southeastern US estuarine biotope (brackish) and the smaller as a local rocky stream biotope, but that doesn't help you much as you probably can't get many of the fish I would put in them. Still, local biotopes are something to think on. Not enough aquarists try them.