Either via FSUIPC's programmable interface (i.e. from a program) by writing to the Sim Rate offset (0C1A), or by using the FS control "SIM RATE SET" with a parameter of 0. The latter can be assigned to a keypress or joystick button in FSUIPC's user options.
Regards,
Pete
Maybe here's the right place to post my Q's...
I want to do the same, syncing multiple FS's, but mainly because I found a set of really good gauges (from RealityXP) which I want to run remotely on another computer. When you run WidevieW or FS Manager the position gets synchronised but no information regarding panel data (e.g. altimeter setting, indicated airspeed, engine information, etc...) is displayed. FS Manager claims to sync this data and its debug log shows it sending engine data packets. But the engine instruments are NOT updated. (the other instruments, dependent on position and orientation, like attitude, altimeter, DG etc are updated). Is this caused by the pausing of the simulation engine? Or does FS Manager have a problem synchronising? I reckon that when you set an engine's N1 value via FSUIPC directly (after receiving it through the network from a server program) that it should change on the screen.
Edit: Success! When you put the simulation engine in its slowest speed through FSUIPC (write a 0 to 0x0C1A) you can basically fiddle around with every variable in FS as the engine runs too slow to ever touch it before you will. Worked for me on the engine N1 through offset 0x2000... does not seem to work through the 'classic' offsets though (in the 0x0896 range)... only the doubles work.