Note that some graphic cards can conflict in the BIOS, when they both try to grab 'default', and then the computer never starts up.
There is also a possibility of driver collision in windows, which is actually more likely on vista/7 then on xp. No idea about linux.
As this is a very unusual usage, you probably won't find help on the usual tech sites either. So You'll never know without trying I guess.
Ok so i just started mining today I have ... hd 6850 ... and im getting a average of 200 mhash/s is this normal my core clock is 955
200 mhash/s looks about right, for a non-optimised setup. See here: https://en.bitcoin.it/wiki/Mining_hardware_comparison