normally you would plug both monitors into the main card, the cards don't need to be connected as Crossfire uses the PCIe slots to communicate between the cards
first thing you should verify is that your motherboard supports crossfire.
once your over that hurdle plug both screens into the main card split if you must, everything elose should be set up in windows via the advanced display tab
You need splitter only if you want to duplicate image on those additional monitors.
If you want full control on all displays, you must plug additional displays via separate cables (HDMI for example). One per each video card. You must disable Crossfire in this case.