You might try _SCREENMOVE -1200,0, for example.
If your main monitor is to the right of your second monitor, I have seen cases where the second monitor is assigned negative coordinates.
For example:
Left Monitor (-1281 to -1, 0 to 720)
Right Monitor (0 to 1280, 0 to 720)
(Assuming both monitors are in a 1280x720 display mode.)
I’ve currently got 2 monitors and an old TV hooked to my main pc, and that’s how their coordinates are set up. Left is negative, Middle starts at 0, and the Tv is +that.