Not that a computer should not have two protecting programs....but a computer should not have two anti virus programs installed and running at the same time.

1) Each software designer has it's own version of proprietorship rules and regulations. This fight between default programs is common among like programs that are constantly running in the background. Thus one program causes conflicts between the other and can cause lock ups...slowness..and reboots.
2)The biggest fault is that each program will constantly show false positives from the competitions program and then the system becomes unreliable at detecting the real culprit. Although there are ignore options these don't always work as they should and before you realize the A program deletes something found in the B program and vice-versa and then neither program is working properly.

Common rule is to have only on anti-virus program running.