The problem with pulling the calf off the cow and allowing it limited access twice a day... is that hungry calves are very likely to gorge themselves and the next thing you know is you'll be treating scours. In order to avoid that, you'd then have to figure out how much she's giving from one side, and milk her out just enough that there's approx half a gallon per feeding for the calf. On the other hand, not only will the calf will self-regulate how much she drinks at any given time if she has the opportunity to drink all day long, but calves can handle more milk in any given day if they can have it "little and often."
I'd be inclined to leave the two together (and add an extra calf!), but you MUST make sure that cow is milked out completely twice a day.
Lance, not to be harsh, but I think you didn't realize what you were getting yourself into when you decided to calve out a holstein heifer. I have no idea what the genetics of your heifer are, but there are fresh heifers out there that will -- with good feed -- reach 80-90lbs or better (1 gallon = 8.8lbs) when they peak around 30-45 days in milk. I have an 8 y/o holstein nurse cow at home that I send to the neighbors dairy for a few months when she freshens, because she gives twice as much as 4 calves can handle -- and I have never fed more than 10lbs of grain/day.