I found that you burn MORE calories by exercising for 60 minutes in one shot than for two separate sessions of 30 minutes.
For me (and everyone is different) I burn approximately 200 calories in the first half hour, and *300* in the second! Yes, I burn half as much again by keeping going!
I can burn more calories by doing an hour today and taking tomorrow off than by doing 30 minutes each day. Or I could work out both days, and burn even more!
Remember, the first 15 minutes are the toughest!
I use a heart-rate monitor to calculate my calories burned, as it's much more consistent than the numbers on the machines, and will account for the effort I put in on any given day (which the SP numbers don't). My preferred exercises: running, walking, elliptical, swimming.