I would work out a way that only one tank at a time would be filled.
The empty tank will not gather condensation , and could be usedonly when needed for range or winter heating.
If you love bestitis , if the upper tank could be used as a gravity feed day tank , it should add to the value of the vessel.
Installing plastic tanks takes time , as the virgin tank must be filled with diesel to let it expand for a couple of days , then secured in.
Pro boat builder will explain the required techniques.