Open Regedit.exe for Key Computer\HKEY_CLASSES_ROOT\RDP.File\shell\
I created a new Key "Single Monitor" and put a new Command Key there like the existing for RDP Files.
The Command Key contained a string with name "command" and Value:
powershell.exe -windowstyle hidden -command "rm app(*).rdp; (Get-Content "%1").replace('use multimon:i:1','use multimon:i:0') | Set-Content "%1""; mstsc.exe "%1"
This deletes existing rdp files called "App().rdp", replaces the Multimonitor Parameter and opens the Profile.