What's your setup? Do you actually have 2 separate X displays, or are you using Xinerama to combine the two?
If you're using 2 displays, they should have their own -display or DISPLAY env variable. You can use that to execute a new app.
$ echo $DISPLAY
:0.0
$ xterm -display :0.0
Open a shell in the other display to get your other display env variable.
There's also -geometry, in the format of [Wth,Ht+X+Y]
$ xterm -geometry 132x80+500+0
When Xinerama is enabled, your other display is usually a +X+Y offset.
Of course, this does require the app to process command line arguments. It then passes those off to X when initialising the window. For xterm, width and height are units of text characters, where X/Y are screen pixels.