i dont get it. you want to use both monitors for the game but have the game on the bottom?...
He has 2 monitors in a vertical orientation. When he makes the game extend to both monitors, the "center" or the "crosshair" ends up between the 2 monitors (This will also happen on Horizontal orientation).
What he wants is the center of the crosshair to remain on his primary monitor and extend the game into the second monitor
Like I said before, as far as I know there really isn't anything out there that allows you to do it. Even with Nvidia's/AMD control center it will still center the crosshair in the "middle" of your monitors. Now if you had an odd number of monitors (1,3,5,7,9, etc) then ya its gonna center it just fine.
TL;DR- Game has to come with that feature, otherwise your stuff out of luck.
EDIT
The "center" of the screen -- where your cursor in blockland would be -- is in the middle of the bottom screen, and the top screen just gives more skyward view.
You beat me FFFuuuuuuuuuuuu