A circular needle is just a long, flexible DPN. You can use two of them instead of 4 or 5 DPNs and get a better result for fewer needle changes.
Two circs in a longer length lets you work both mittens (or socks) at the same times which means no need to count rows because you work half a round on mitten A and then half a round on mitten B (from a second ball or from the other end of the same skein). Then you turn your work like for a DPN change and work the second half round of B then change back to the yarn for A and finish the second half of the round on A.
It may sound confusing but it really isn't.
I suppose you could get a second set of DPNs and cast on to both sets and just rest A while you to a round or two on B. Then it's just taking turns between two project that happen to be the same pattern and using the same yarn (two skeins/balls or two end from one skein or ball).