#By the rules - probably not, but a DM might allow it as a houserule


The rules say that while you are wielding the shield you automatically get the bonus to your AC. So while you are holding and using the shield you are assumed to be getting that bonus at all times. There is no mechanism in the rules for turning this bonus on and off.

If you want to say that your character is foregoing that shield bonus, say by flinging your arms wide open so the shield is not protecting you, then you must ask your DM if they will allow that to reduce your AC for that attack.

If the DM allows it as a houserule, then the could probably allow you to block one attack and then leave yourself open for the next one, Though they might also rule that once you leave yourself open that you are vulnerable to any other attacks as well. Since this is not specified in the rules it will be up to them entirely as to how to manage it.