What determines the primary display in a dual display desktop setup?
Solution 1:
Usually on graphics cards, primary display is DP > HDMI > DVI > VGA. If there are muliple DP, the first one is usually the one furthest away from DVI and HDMI.
Also, check your cables. Some cables are faster and may get signal to the monitor sooner.
As well, check BIOS to see if you can change settings for monitor.