Re: U3818D, two sources, PBP picture fill not out 1/2 of display
I do think that DDM software will help positioning app for 1 computer.Since you are using PbP, you need to find find a driver for both PC so you can change the resolution to 1920x1600 so that both can feed the screen on both side of you screen. (check Nvidia, ati, intel driver and or their forums.
By default each computer will set their resolution to what the screen can do so (3840x1600).
The scaling should be adjusted in the video card GPU settings, not the Windows Display settings.