There are very few limits, it all depends on what you are trying to achieve, if you are looking outside the box to coin a phrase then it is hard to accomodate everyones personal preferences, although i think GHL do more than any other manufacturer to try and achieve this.
I think as you get to know the unit better you will find very few limtations.
If you are running two tanks as per your thread suggests then maybe look at the future and networking two profiLux's via the RS485 expansions. They will run independant from each other sharing a common software program.