Over and above what has been said.
Resident AVs load low lever drivers that hook files so they can be scanned before they are run, these can clash and cause a conflict, which could lock up your system. If this happens during boot then you could possibly be locked out of your system.
You may well be able to get in through safe mode but then you have to start resolving the problem with will probably need at least one AV to be uninstalled, you may even need to uninstall both and run other tools to ensure all remnants are gone, before reinstalling one AV.
There are a lot of could, may, etc. in the above but I for one even with a good back-up and recovery strategy will only have one resident AV on my system.
Disabling the other AV simply isn't enough and Symantec/Norton is probably high up on the list for possible conflict with other security applications (even when uninstalled needing other tools to completely remove), you only have to browse the forums to see the detritus left behind after a NAV uninstall.