after some time of tricking, I'm now with 1.1.9.3, not my first time with (U)VNC but first time I'really using it with the need for detailed control of two monitors. The final goal is to get a presentation setup where I have a two monitor setup and want to show additionally only secondary screen using RaspberryPI VNC client. So I need to start UVNC server with secondary screen as default (UVNC settings, checkmark at secondary, none at primary). My setup is a left secondary, both screens have 1280x1024. I'm testing with UVNC 1.1.9.3 client on a laptop, not my final target, but fine for testing.
Even I select only the checkmark for secondary in settings, I get a full 2560x1024 display on my client when connecting. Using the "Select Full Desktop..." button in client allows me to cycle thru all modes, screen1+2, screen1, screen2 (actual sequence may be different), so the mirror driver is active, which is also confirmed from pressing the check button for the mirror driver (1.00.22). I also checked the ini file, secondary=1 primary=0, no further similar lines.
I tried to do some wireshark tracing for the RFP and had the impression that with the initial connect the server announces the 2560x1024. So my feeling is that with the secondary left fix the initial calculation of the width was not updated in case of default for secondary left.
Please check, Thanks
NB. I had a lot of trouble understanding an finding how to use all this multi monitor stuff, is there somewhere a comprehensive write-up for the current version, how to setup and configure ... and how to verify operation

knuvnc
Short update: The sequence when connecting the client is for two screens, secondary left for default: after connect:2+1 , next switch:2 , next switch:2+1, next switch:1, next switch:2, next switch:2+1, next switch:1 and so on. Switching is by clicking the approriate icon in UVNC client.
(graphics: NVIDIA FTX460 driver 9.18.13.1106 18.01.2013 / Win7 Prof)