this could be tricky to test if you dont have two can hooked up to computer but im only using a firewire cam and the webcam for now .... but what i am try to achieve a cross fade between two live cams ....
pls find patches attached

I open PD.... I open the cam mixer .........I open the main ...i create gem window .....
and i can see the cross fade occurring in the output window (bottom left of GEMwindow but as soon as i adjust the slider it reverts back to webcam only...?

ive check the slider properties , order of opening sequence .....and am a little stumped any ideas ?

L.E.S video mixer.pd
L.E.S. main.pd

@WilliJ Hello Will...
I cannot check your patch because my webcam crashes Pd...... but....... your crossfade slider is sending values between 1 and 100. [pix_mix] expects values between 0 and 1 and that could be the reason?
David.

Well spotted Thanks man @whale-av