I don't see a problem if they are both left footed. Most clubs play with 2 right footed CBs, and have no problems, why would 2 left footed CBs be a problem then.
Marquez-Puyol, Pique-Puyol, Pique-Mascherano, all were right footed and were great CB partners, the same as Pepe-Ramos, Varane-Ramos...