Someone with more knowledge than I can correct me. IMO this is a tough question.

In one respect your body only begins to burn fat after roughly 20 minutes of cardio. Meaning that you are techinquilly only burning fat for 10 minutes each time if you do two sessions. Where as 60 straight through will get you 40.

On the other hand. If you do cardio for 20-25 minutes your metabolism will kick into high gear as your body anticipates a need for increased energy, your body will stay that way state for a few hours afterward. The amount of time in that state does not change in relation to how long you work out. So 1 60 min session you get higher metabolism for say 3 hours. 2 sessions at 30 minutes you will get higher metabolism for 6 hours if you space them out.

I would guess it could be a wash? I'm sure someone else can provide better more defining info.