Arkathan said:
Error: Having a separate one for the two added together was the only way I could think of to avoid sticking a 50 ton CFT and a 40 ton DFT in a 60 ton cargo bay.
While it may be stupid, it is perfectly legal.
You may have a Collapsible Tank that can only be filled if something else is removed from the cargo hold, or even if the hold is expanded by a module.
The only hard limit I can see is that the empty collapsible tank and the demountable tank must fit at the same time. Of course you can't fill the collapsible tank unless the hold currently have enough space free.