I previously use RealVNC and toggle to UltraVNC (great job).
With RealVNC, I have create shortcuts files (I put on the desktop) to my remote machines.
In these text files, ie FileName.VNC, a parameter named "Monitor" stores the screen number to show the viewer window.
UltraVNC doesn't care with this parameter and the viewer windows always appear on my screen numbered 1 (left screen for me).
It could be a good idea to solve this simple problem.