In concept, your doubling up of antivirus programs is good; however, today’s antivirus programs cover both viruses and malware.
If you don’t notice much of a performance problem, it’s probably okay, but in general, Microsoft suggests only one antivirus program to be installed as a full-time protector (rather than on-demand when you choose to scan).
From Microsoft:
http://support.microsoft.com/kb/189264/en-us
Look at the last line in the Summary portion.
Mine are set to run scans at different times. So far no problems with performance.