See parent post for the joke. It's a pun on their name, Kagamine, and Kagami Mochi. Their heads are stacked like mochi. Coincidentally, Rin's character item is an orange.
Updated
Alright, Len-san. The new year is just a bit longer, isn't it!Seems soAnd that joke will be in just a bit longer.That joke?The yearly tradition. With the Kagamine of Kagamine for Kagamine. It's that one-liner! I also brought an orange along!Ohhhhhhh
(So we're doing it this year too?)Look forward to it!
Don't fall asleep and wait a bit longer!