Two anti-virus software on a single PC

A couple of days ago I asked an IT friend about why two anti-virus software packages could not be installed in a PC. The answer given to me was that each anti-virus software has some codes resident in memory to protect the kernel and important system files. If two anit-software packages are there, they will clash with each other. This is very much like the case of not allowing two DHCP Servers on a single network segment. I believe the answer really makes logical and technical sense.