Also consider that they won't accept "but I was snowed in" as an excuse for missing classes. You *could* live in one and commute to the other, but you'll need to have a place you *know* you can stay if there's a forcasted blizzard, 'cause that'll shut down the roads.
Be aware that the side roads get shut down pretty regularly in the winter, and that the traffic on I-25 is pretty nasty in the Denver direction pretty much whenever people are awake, and twice as nasty going to denver in the mornings, and coming from it in the evenings. There are towns pretty much continuously along I-25, all commuting to denver. *I'd* live in ft. collins, personally- I think it's more affordable than boulder, and grad school has a slightly more forgiving schedule.
Commute wise, tho- you'd be better off living on the outskirts of boulder- so that you're both going opposite the major traffic to and from school.
Or each living in your respective town and "visiting" on weekends.
j.