I personally choose to split them so that they are about even level midway in the tank. For all my canisters actually. 2 on any tank, well. Try it out to your own likings. Inlets in the middle, returns on the sides. Opposing filter inlets and returns on each side, i.e. FX5 #1 inlet left, return right FX5 #2 inlet right, return left. Or both returns on one side and inlets on the other. Trick I would imagine is to create a better flow than just 1 FX5.
Realistically, if your filter is packed with media. 2 500g biomax, 750g prefilter, blue "fine" filter pads and the foam blocks. I can't see a need for another filter other than chemical filtration. One packed right should be enough for mechanical and biological. If you're just looking for flow, I would think a powerhead might be cheaper.