Q:

Is a double-challenge possible?

In American football (NFL), is it possible for both teams to challenge on the same thing for the same play?

A:

No.  Once one team challenges the play, that challenge covers all reviewable aspects of the play, at least as of the most recent rules (2015).  From Rule 15, Section 2, Article 3:

Note 2: All reviewable aspects of the play may be examined and are subject to reversal, even if not identified in a coach’s challenge or if not the specific reason for a Replay Official’s request for review.

Thus, one review covers all possible reviewable actions.  A second flag is not explicitly discussed explicitly; however, it would be unlikely that a referee would allow such a challenge (and if he did, it would almost certainly just be a wasted timeout).

