Which version of Kopete do you use? I use 0.12.2 and it works fine (with both webcam and tv card).
A possible solution would be to set up the kernel module for yout tv card in a way that it does not assign /dev/video0 to your tv card, but /dev/videoX, so your webcam will be assigned to /dev/video0 when connecting it to your PC. That should probably solve your problem.
I user the module bttv and I would have to load it with the option video_nr=X
This way, the device node for the tv vard would be /dev/videoX.
Find out which option you would have to use and then set up your modprobe.conf according to it (or better: set up your modprobe.conf.local).