The UltraStudio 4K is found by Resolume as a single device, you can not dedicate SDI A as one monitor and SDI B as another and due to this it won't work. I've spent days trying to figure this out and my findings are:
The US4K as 1 monitor out with 2 slices and one of which has "is Key" enabled in advanced on the output tab of one of the slices. Here is the thing, this look correct in Resolume as you have the bottom slice as alpha and the top slice as the viewable Fill but it still goes out on 1 output. There is no config available to split the 2 chanels over the 2 SDI outputs and as the signal goes out equal summed and ready, Key & Fill don't work as it should. Premultiplied, straight doesn't matter as far as I know.
I used both my BMD Mini Monitor and that worked with perfect result as I could config them as 2 outputs where I set one as the "is Key" which I summed in the switch but there was sync issues. When I did the same with 2 US4K units and synced then on ref input is worked like a charm. But still thats just bloody anoying to use 2 US4K for a simple task like that.
So basically the Resolume team have to add the feature to set up SDI output on units like the Ultrastudio rack decks if this is to be done by a single unit