As others have said, there is no apparent reason why you couldn't/shouldn't do this. There are likely several different ways to plumb it, that'll accomplish the job.
One thing that occurs to me, is you may want to plumb a connecting line between the tanks so they self-level. Otherwise, one tank may consistently fill up before the other. Being as how there is no solid waste, you wouldn't need a big line to do it -- 1" would probably be plenty.
And you might also think about making the mod reversible, in case you ever want to put it back the way it was, for resale.
If you do it, some pics and an article about the job would be interesting -- I'd enjoy following the work.
