Mw13
“it is good enough to give every person a small piece of the top and middle matza, and they can eat the rest of two kezaysim from other matza”
If you do it that way, there is no reason to have two kezeisim. The reason we eat two kezeisim is because safek which matzav is used for mitzvah whole or broken so we do both.
But if you’re eating other matzas what is accomplished by eating two kezeisim?