As the saying goes, if you ask any two Jews about something, you'll get three opinions.

In the days of the Temple. one of the greatest Rabbis of any time, Hillel, would eat the maror a little differently. Since the actual commandment in the Bible say that the sacrificed Paschal Lamb should be eaten with matzah and maror, he would make a sandwich of some of the lamb, with a bit of maror, between pieces of matzah. Since we don't sacrifice a lamb anymore (hence the aforementioned chicken neck on the Seder plate), we just eat a bit more maror between two small pieces of matzah.

Question: Why is a sandwich not known as a "hillel," even in Hebrew? Because languages are weird.

Once we've passed around the matzah and maror, there's actually no additional blessing for this, since it's another way to fulfill the previous blessing. We do it both ways, just in case.