You should not run more than one av (or firewall, or spamfilter…) in the computer. It’s not a matter of the manufacturer or trust… You must trust in the av you are using with the limitations that nobody is perfect.

You can, or better, you should have a layered defense, I mean, different programs doing different protection tasks in the computer… One for each job…

The main reason installation fails or systems freeze when using new AV programs is the inablility of the old ones to uninstall properly. I have had to dig up removal tools for Norton before anything would operate properly. This is not an avast issue, Kaspersky, McAfee and even Grisoft (e-mail plugin) have their own unique uninstall issues as well.