I find it highly unlikely that Ferrari would have both Alonso and Vettel onboard. Modern F1 has been about having one strong driver and a second driver in support -e ven if it didn't always pan out in that order (and sorry Webber and Vettel *do* fit into this category). I'm struggling to think back in recent history to when a team had two top rate drviers side by side.
2007 - Alonso and Hamilton, but Hamilton back then isn't Hamilton of today.
2006 - Raikkonen and Montoya, until one of them took their bat and ball and went home.
I think its a successful strategy to be used, as it usually results in one driver benefiting from the developments, team orders etc, and the second driver picks up the scraps as a bonus.