Anti-Virus programs take up an enormous amount of your computer's resources when they are actively scanning your computer. Having two anti-virus programs running at the same time can cause your computer to run very slow, become unstable and even, in rare cases, crash.
If you choose to install more than one Anti-Virus program on your computer, then only one of them should be active in memory at a time.
There are basically two types of these programs:
On-Access and On-Demand
On-Access Scanners
As the name implies, are scanners that run in the background all the time the PC is turned on and running. The main function of an On-Access scanner is to monitor activity on your machine.
On-Demand Scanners
As the name implies, are scanners that only run when you ask them to.
Such as:
Online Scans and scanners that run on your machine but are not actively scanning your machine