If I understand this correct, you have 2 windows clients on the same pc with 2 monitors.
And the problem you facing is that it is only displaying on one monitor. I have faced this challenge before and did allot of reading and only when you face and solve these issues do you realise how awesome xibo really is.
Ok the issue you facing can be resolved by creating custom profiles for the screens. The default profiles are Android and Windows.
So create new profiles e.g. Left and right
The reason for this is that you are going to set the resolution and the co-ordinates of the displays.
Lets say you using a resolution of 1920 x 1080
So profile left will be set to 1920 x 1080 and the left pixel will be on 0.
Then profile right you will set the resolution to 1920 x 1080 and the left pixel to 1920
This is based on your display settings assuming the left display is your primary display, and the display settings is set to span.
NB: Depending on your setup it is advisable to set the resolution and not leave it on 0 as it will take up the entire screen. I had this issue when I used eyefinity and the client that had the resolution set to 0 (Which means max resolution) would take up all the space.
Hope this helps, all the best