The Biblical verse, `They shall eat it (the paschal lamb)
with unleavened bread and bitter herbs" (Numbers 9:11)
is probably the basis for the use of chazeres as one of the symbolic foods.
The verse speaks of bitter herbs (plural) and not a bitter herb.
Therefore, two types of bitter herbs (vegetables that have a bitter tang)
were introduced.

Source: The Family Seder, Prepared by Rabbi Alfred J. Kolatch.