I tested a few minutes ago.
Stop service before any changes made to ultravnc.ini (and some full reboot for added test reliability)
Primary=1 // Secondary=0 => Works as expected
Primary=1 // Secondary=1 => Works as expected
Primary=0 // Secondary=1 => Doesn't work (same behaviour as p=1/s=1)
Also android viewer fails to connect (really disconnect just after showing desktop) but only if try to connect after logon.
(Android viewer problem isn't a big deal right now; only FYI)
XP Home (windows updates to date)
Nvidia Geforce 4MX dual monitor setup
1680x1050 + 800x600
Mirror hook driver fully uninstalled, re-downloaded and installed again previously
2x Android viewer (800x480 and 480x320 screens)
The same xp machine (loopback view only 50% scale for testing)
If do you need more data or further testing, don't hesitate to contact me.
path=C:\Archivos de programa\UltraVNC