Hi Ritva

My guess is for twin papyrus stems twined together. We see them right from the 4th dyn - New Kingdom

On 4th dynasty palace facades the two outer parts to the papyrus head are rounded and they remind us of either the decorative plants on the Narmer label or the Mesopotamian palace facade which is dated to the Jemdet Nasr period. Emery referred to the Jemdet Nasr decoratives as "two lotus flowers tied together". During the last two centuries though, for want of a much clearer distinction between the two, egyptologists often used the term "lotus" for both waterlily and papyrus decoratives. In Egypt, however, the waterlily or lotus has always been discernable from the papyrus by virtue of its large triangular shaped petals. If there still remains doubt it is possibly due to the way the top of the umbel was drawn (rounded) in Early-dynastic times for the similarity is obvious when we look at the lotus hieroglyph Sha. On the papyrus the extent of the basal sheath in relation to the umbel is important and on the waterlily it is the triangular shaped petals which extend from the bottom of the flower to the top.

There has always been confusion between lotus and papyrus.