Trying to run the real time portion of more than one anti-virus program at the same time is asking for trouble, but there should not be any problems having more than one installed. To avoid conflicts, you would probably have to disable the real time scanner (if you use one) before running an on demand scan from a different program.

Having multiple anti-virus programs offers better protection, since for any particular virus, different programs may be updated faster or handle cleanup better. Also, since some viruses try to disable anti-virus programs, having multiple programs gives you a better chance of at least one remaining operational (either because it was updated to counter the virus, or because the virus only targeted the most popular anti-virus programs).