A: I don’t know if any independently tested, quality antivirus that doesn’t recommend against doing this. For good reason…
When a file/application opens, it is the job of the antivirus app to scan it before anything else (like the system or another antivirus app) gets access to the code. You can see that with more than one antivirus app installed, you have created a conflict – which will scan files first.
You didn’t mention what you are attempting to accomplish. I’ll assume it is better Protection than any one AV brings to the table.
A quality AV will catch perhaps 99.9% of KNOWN malware. Depending on whose stats you choose to believe, there are up to 40,000,000 malware. That would still leave up to 40,000 that NO AV will catch.
The solution is to combine a quality AV with application whitelisting and logging in with a non-administrative account. This will almost entirely eliminate that threat vector.
But don’t worry, there are many other threat vectors still out there – network eavesdropping, email, texting, and other communications sent without end-to-end encryption, zero-day attacks, more.