What are you wanting to put in the sump? Heater like you said, but more biofilter media? cause you could use inline heaters if that is all you are really wanting to use a sump for.
Sumps are great for increasing overall volume, just wondering if you really need to do all the work for a sump if you have two FX5's
* i suppose on the issue of evaportion a sump could help, assuming you don't let it run dry. Also ave you considered materails around the tank being waterproof or good ventiltion?