I don't know how aggressive mantis shrimp really are, but I am guessing that if they are able to see each other then there will be problems, meaning if you want to divide the tank then you would have to use a divider that isn't see through. What I would do is get a single mantis shrimp and have a sort of "main" focus in the tank instead of having two different shrimp with different needs.
My tanks (so far
5.5 gallon Planted Dwarf Puffer tank (2 Dwarf Puffers and a lot of RCS!)
16 Gallon Planted
(GBR pair, 3 amano shrimp and a whole bunch of snails!)