It should work correctly. You screenshot shows Monitor “: 3840 x 1080” which correlates with what you just said. Two monitors/beamers combined. The game should render at 3840 x 1080.
The greyed out Resolution box is confusing. Yeah. It is not showing the current setting but some older value from before it was set to borderless windowed. The game will use the “Montors”, in your case Nvidea Suround created Monitor, native Resolution.
A good test is to take a screenshot ingame with F11 and then look at its dimensions.
You can find Screenshots under: C:\Users<YourUser>\Documents\I-Novae Studios\Infinity Battlescape\Screenshots
Also you can post them here so we can see how that looks. Obviously most people don’t combine two Monitors to play because of the bezel but beamers can work of course.