Musar zu Wajikra 18:18

Having sexual intercourse with two sisters [while they are both alive, Ed.] is perceived as equivalent to damaging the equilibrium in the domain הוד ומלכות. Only our patriarch Jacob, whom G–d Himself had called "אל" a celestial being (Genesis 33,20, cf. Megillah 18a), attained a level of spiritual perfection that permitted him to use the sceptre normally only used by the "King" in the Celestial Regions. In connection with the prohibition to marry two sisters simultaneously, the Torah (18,18) uses the expression לצרור לגלות ערותה, which we may provisionally translate as "as a rival." Rabbi Abraham Saba, in his Tzror Hamor understands the word as "creating a knot," i.e. tying up Celestial conduits and preventing them from exerting their beneficial influence on their respective partners in the domains of the emanations, and through them on this world. [This comment of the Tzror Hamor is not found in his commentary on this chapter in our פרשה. Ed.]