QC: Well, with two scanners I generally only ran into issues if both were licensed products, AVG and Norton and the like. Being cheap, I use AVG's free. It doesn't find everything though so I found a separate scanner that helped dig out anything that hid a little better, and that was something that always had to be run manually. Same thing for the dedicated rootkit finder, it could find things AVG missed and was something I used if I happen to get nervous with my system, another manual program. Maybe Avast and Norton actually are that complete with their scans but I've not come across something that always got everything. The ones I used also never noticed each other which was a good thing.
You won't find that you're finding things with two scanners that you wouldn't with one good scanner. All you're doing is adding load to the computer and providing a convenient place to hide malware.
In practice, the only thing you really need antivirus software for is drive bys from various websites, but with No Script, Useragent and a few other add ons you can handle that as well. In the 15 years I've been online, I've only caught a virus once and I haven't had one since probably 1998 or so.
Ultimately if it's hidden good enough to trick a good antivirus product, it's probably not going to be found by another one, the main risk are the unknown malware.