I tried to look at pics to see what the back panel looks like, no luck. I looked up the manual and what I though was a simple question from you turns out to be a good question. The manual says:
"When the GMX1200H is used with a single BEHRINGER ULTRASTACK BG412V, the rear panel switch must be in the stereo position and the two outputs should be connected as shown in the figure" (figure shows two cable going to the cab, one in the right, one in the left inputs, switch is in mono)(?).
I'm not a gear pro so I don't know how the whole stereo/two cabs thing works but what I do know is that usually you have a external speaker output(s) on the amp that either says "8ohms" or "16ohms" (or has an ohms selector switch), so you look at your cab and, say it's mono input is "8ohms", then you just use the correct 8ohms output from the amp (weather it be an 8ohm output jack or a switch that you select to 8ohms) pretty simple but, yours seems like it's supposed to use both speaker outputs into either one or two cabs.

I would suggest trying one speaker cable from one of the outputs into the mono (main) input of the cab. Just make sure the ohms match (it's ok to mismatch ohms but make sure it goes from lower to higher..ie..8ohms from the amp to a 16ohms cab, but there will be a difference in tone). If it's still not working, try the other output and try the switch in both stereo and mono (none of these tests will hurt anything, but get a second opinion before trying two cables into one cab). oh...and, MAKE SURE IT'S A "SPEAKER" CABLE, NOT "INSTRUMENT". I'm not yelling, just wanted to make sure you saw that, it's VERY important.
Like I said, I'm not a pro and I'd hate to give you bad advice but I think on most fullstack set-ups you would go: one mono into the top cab (from the amp) then plug the top cab into the bottom.