Mac Pro won't display after switching screens

Ok so this is a strange problem but it's very annoying (at least to me). I just purchased a LG Ultrawide 29" display and connected it via HDMI cable to my Mac Pro (Home machine) and a second HDMI cable to my Macbook Pro (office machine). Both the Mac pro / Macbook pro work well with the large screen, HOWEVER, when I am using my Mac pro and select the onscreen toggle to switch to HDMI 2 connection, it works and switches to my Macbook pro. But when I try switching back to HDMI 1 (Mac Pro), it just comes back with a black screen and says no signal. Even if I replug the HDMI cable it doesn't work. I've set the sleep timer to 3 hours, still doesn't help. I know it hasn't gone to sleep because itunes can be playing when I switch and it remains playing music all the way through and continues when I try to switch back. The only way I've been able to recover is long press the power key on my Mac Pro and restart.

The only reasonable difference I am aware of is the Macbook pro is running Yosemites, while my Mac Pro is running Beta El Capitan. I have a feeling this is probably the cause, but just wanted to know if anyone else has solved this in some way.

Thanks