They are normally used together to create one tank.
But sounds a good plan to use 1 + 1.
Do you have a diesel heater? Or are thinking of getting one installed or might possibly do so? The normal location for this is right where the tanks go, so I understand.
If you do go this route, personally I would go 27 fresh / 21 waste, presuming you are happy drinking from the tank.
We have 38 fresh / 17 waste, and have never filled the waste before needing to refill the fresh, mainly because so much of the water goes into our teapot!