btw: @waking: didn't fully get your ABX explanation, bit confusing... ;)
Well, apologies if it wasn't clear enough. Let me try one more example.
Consider these hypothetical products:
Brand "Y" AV includes anti-spam/phishing.
Brand "Y" FW does NOT include anti-spam/phishing.
Brand "Z" AV does NOT include anti-spam/phishing.
Brand "Z" FW includes anti-spam/phishing.
If you install Brand "Y" AV & FW you have protection
against spam/phishing.
Similarly, if you install Brand "Z" AV & FW you have
protection against spam/phishing.
If you install Brand "Y" AV & Brand "Z" FW, you will
have *double* protection against spam/phishing.
But if you chose to install Brand "Y" FW and Brand "Z" AV,
you would NOT have any anti-spam/phishing protection.
So there is a (at least theoretical) risk that you *may*
be missing some protection if care isn't exercised when
combining AV and FW from different providers.