It would be similar to if Hamas and Meretz got together and agreed on peace terms. Israel would not accept Meretz' agreement because Meretz is not the recognized leadership of Israel. Abbas term expired long ago. The most recent elections were handily won by Hamas.
Knesset passes 'Norwegian Law' allowing ministers to resign their seats, remain in cabinet (Haaretz)
from the article: Hamas: Direct Israeli-Palestinian talks illegitimate, coerced by U.S.