There are but a few breakers in the grand scheme of things who's terminals are labeled as being suitable for the connection of two conductors. As Jerry said/implied, unless you popped that breaker out of the panel and read every printed label and every embossed word, I would not assume that two conductors are appropriate.

On the other hand, the apparent duel conductor inlet openings pictured does tend to lead one to believe that perhaps this is one of the few particular breaker designed for two conductors. Even still, I would want to see writing to that affect on the very breaker with my own eyes before I let it go.

I would not however (within the scope of a generalist's visual inspection), attempt to remove the breaker from the panel just so I could read its every marking.