Is it ok to run Windows Defender while using avast! anti-virus software?

One of the “rules” I’ve always understood but never really tested is to be sure that you only have one anti-virus program running at a time. I’m using avast! on my Window 7 PC, and I have started using Windows Defender too. I haven’t seen any issues. Should I stop using one of them? What will happen if I continue to use both?

Story added 19. November 2012, content source with full text you can find at link above.