Rabbi Aryeh Lebowitz

Lechem Oni is Madak or Shalani.

But Prusa should be eaten from the broken piece.

And to eat the Prusa first is also not correct.

Because the Shlema is on top of the Prusa.

So in Mavir and Alemitzus, you shouldn't bypass the Shlema in order to eat the Prusa.

And maybe that's where the practice comes from.

To eat them both simultaneously.

Shulchan Aruch writes in Tavai and Esef Aleph,

that you eat them b'aseba b'yachad, k'zayis m'kalechad.

In Kitza Shulchan Aruch, Kuf Yotah Sefei, he explains, you're b'otzea min ha'alyona, you cut off a piece from the top one, and also from the prusa, k'zayis from each one, and you give a k'zayis from each one to everyone sitting around the table.

So k'zayis of the top one and k'zayis of the bottom one to everyone sitting around the table.

Now the obvious problem with that is, most often,

it's going to be hard to find, especially in the Prusa, a kezias within one broken piece of matzah for everyone around the table.

How big is this matzah, even if you have a small shir of a kezias?

Rosh HaKadosh Baruch Hu pointed out that, you know, this was the Melech, he said his father-in-law used to eat part of Shemesh Kab Seder.

and Rav Shimon Shkaab had a seder, would take the matzah, and he'd break off five or six kazesim for everybody at the seder.

He says, what?

Five or six kazesim for one matzah?

Wasn't he about my face that the matzah was growing?

No, kazayis is not as big as you think it is.