Mercury Transmit only on primary monitor

I have been searching for the answer to this question for over a year and YOU JUST SAVED MY LIFE THANK YOU!!!!

I know this thread is like 3 years old and there is a solution to this, but I think I found a better solution if anybody wants Displays have seperate spaces

ok so the problem I had was I had about 4 spaces ( virtual desktop) open and I usually work on desktop 2, my secondary screen is labeled desktop 4 BTW.

whenever I enable mercury transmit to my 2nd monitor, it appears on my primary monitor.

yes, disabling displays have separate space did "partially" solve the problem. I say partially because if you use a full-screen app/window, this will render the 2nd monitor useless. 

so the workaround was I created a new space and moved whatever was on Desktop 2 to the new space and deleted desktop 2 and this fixed the problem for me.

I think for some reason the system recognized desktop 2 as monitor 2 which is a weird behavior because that would mean I would have up to 4 monitors listed in mercury but there wasn't.

well anyway if anyone needed a solution without disabling displays have separate space, here's your solution. 

I've attempted to duplicate your setup and had no issue using either my monitor or my laptop as the display. I did notice you have an AirPlay display selected under your Display preferences in OSX. If you set that to off, does that work?