Depending on the DVD, you can see where a workout would naturally break up -- for example, they might do standing poses and then go to the floor for backbends -- so you could do the standing pose section one day and then the backbends the next day. Since most workouts have some sort of a WU and a relaxation section, try to work those in too so you don't turn on the disc and jump right into something you're not ready for. If you have to cut out savasana, you can lie quietly on your own after you turn off the DVD.
I often split up my longer yoga DVD's -- there are a lot of DVD's out there now that have the shorter pre-mixes. Shiva Rea's DVD's have great pre-set sequences, or you can program your own.