How to perform action if two rfid tag UIDs have been detected?
What you can do is store the current time whenever a card is swiped, and then when another card is swiped check how long it's been since the last card was swiped

first create a couple of variables

unsigned long swipedTime;
int tag1 = 0;
int tag2 = 0;
int swipeTimeOut = 10000; // how much time in MS you have to swipe both cards


Next we'll jump down to where where you read the cards.

      if (mfrc522.uid.uidByte[0] == 0xA4 &amp;&amp; 
           mfrc522.uid.uidByte[1] == 0xB8 &amp;&amp;
           mfrc522.uid.uidByte[2] == 0xB8 &amp;&amp;
           mfrc522.uid.uidByte[3] == 0x96) {

               ........

               if (tag2 == 1 &amp;&amp; millis() - swipedTime &lt; swipeTimeOut ){
                   // both tags have been swiped, print serial message here
                }

                tag1 = 1;
                swipedTime = millis();     
        }

        if (mfrc522.uid.uidByte[0] == 0x03 &amp;&amp; 
           mfrc522.uid.uidByte[1] == 0xFD &amp;&amp;
           mfrc522.uid.uidByte[2] == 0xBC &amp;&amp;
           mfrc522.uid.uidByte[3] == 0x02) {

               ........


               if (tag1 == 1 &amp;&amp; millis() - swipedTime &lt; swipeTimeOut ){
                   // both tags have been swiped, print serial message here
                }
                tag2 = 1;
                swipedTime = millis();     


         }


You'll also want to come up with a system for resetting tag1 and tag2 to 0 or "checking them out".