Lots of good advice and experience already posted. I think a lot depends on where you live and what's in your pasture...Dexters are "good foragers," and "efficient converters" but sometimes I think their reputation leads people to believe they need nothing else but what they can scrounge up to survive on. You mention snow...in which case, they'll probably appreciate hay to stay warm. Some Dexters never see grain, and others (mine maybe) get too much; personally, I think in the winter (here anyway) they need full-time hay regardless of whether they're getting any grain.
I've noticed that ours that were on really good summer pasture maintained their condition much better and longer than the ones on the "so-so" pasture.
Depending on the condition of your pastures and your cows, maybe 3/4 cup is all they need to keep them in good condition and coming to you when you call them. Our long-leg "big" Dexter cows nursing calves get about 4# of grain a day and do very well. And some of our short-leg Dexters need the Jenny Craig plan, just on pasture. With just two, you should be able to watch their condition pretty closely, but you may have to stand guard to keep the bull from getting it his 3/4 cup and hers as well. Be sure they have minerals too.
Would love to see a picture!!!