Well my opinion, for what it's worth, is it would be OK as long as the total volume equals the original size tank. Another consideration would be the oil feed, by that I mean there shouldn't be a condition where one tank could run dry & expect the second tank to feed it. Maybe larger feed tubes with a tee to the engine in the middle?? Again, Im no expert, just thinking out loud.
I'm pretty sure there are a bunch of threads where custom tanks have been relocated under the seat etc.