THEY DON'T NEED THE SEALS. That was a mistranslation. The actual translation (the accurate one) was that Sasuke's Rin'negan and Naruto's Bijuu chakra was needed, and in unison, they both needed to do the Rat Seal in order to release the Genjutsu and release the tendrils of the tree holding and sucking the life force of the people within it.