That's why I was thinking if you took one tank, like a 20g long, and divided it, you would have identical water quality on both sides (water flowing freely between the two sides). To really make it the same you would probably want two filters and two heaters, or maybe none at all would be better. The only variable then would be the lighting, this would have to be pretty sealed off to make sure that each side only gets what it's supposed to. I would test it but I don't have any spare 20g tanks lying around, I think it would be too hard to seal off the lighting in a 10g (which is all I have in spare tanks right now) with such a small amount of surface area available on each side.