I have seen some seder plates which contain places for both חזרת (chazeret) and מרור (maror) and other seder plates which do not contain chazeret. On those that contain chazeret, it appears that people place bitter herbs on both spaces. What is the difference between maror and chazeret and why do some seder plates contain both while others contain only maror?
According to some sites that I have seen, the reason is to have two items for the mitzvah of maror (Maror) and for the maror that is used during korech (Chazeres). It seems that it is not that they are different, but in order to have enough for both and to show that there are two different occasions when they are used. This is from memory only as I do not have the exact source where I learned this. Google points to a number of places which mention this.
Romaine lettuce and bitter herbs.
We use both Romaine lettuce and bitter herbs for both Maror and the Korech sandwich.
Note that Chabad.org here assumes that people will understand the term bitter herbs to be horse radish root. In actuality all of the five species in the mishna are bitter herbs so that the Romaine lettuce is a bitter herb.
I have seen some people use Chazeret (Romaine Lettuce) for Korech as it spreads out along the Matzah and horse radish root for Maror. Of course, as @DoubleAA pointed out, the Chazeret is specified first in the Mishnah as the preferred species.
The confusion is that Hazeret is used these days in Israel for Horseradish. hazeret in aramic is lettuce. See Psachim 39a https://www.sefaria.org/Pesachim.39a.9 Rabbi Oshaya said: The optimal fulfillment of the mitzva is with ḥazeret, and Rava said: What is ḥazeret? It is lettuce [ḥassa].