im no doctor, but i dont see why you can't use dutasteride on some days and finasteride on others.
however, i dont think you should "play around" with the dosages, as it'll be hard to tell what dosages will work best for you. i think you have to pick a dosage (and designate which days to take dutasteride, and which days to take finasteride) and stick with that for 6 months to a year. you can change it up once in a while, but id imagine you have to stick with the choice for a long while