If it were me...during the winter a nurse cow at my place has to have four calves on to pay her way. During the summer she can get away with only two...but if she milks enough for four there's no sense in just having two on. If it were me I'd make her take four calves.
I'd cut back her grain - I assume you're graining her? - and pen the calves separately for 12 hours, which'll make them hungry. She'll also give less milk on less grain so they'll be looking elsewhere for milk than the front quarters. Then I'd tie the cow up if you have to...makes them easier to work around, and spend a little time forcing those calves to drink off the back if they want any milk. With four calves, only two can take the fronts...means the other two HAVE to go to the back quarters.
I'm not really sure if it'd work to dry off JUST the back quarters. It's usually "all or none"...although I do recall studies done regarding how much dry time dairy cows needed, and they compared cows with no dry time to cows with quarters on one side dry, other side milking, to cows with all quarters dry - so I'm sure it can be done.
There is a cow on the dairy where I work that has a blockage in the front quarters and only milks out of the back two. She stays REALLY full of milk in the fronts the entire time she's lactating, and only when she's close to calving (when we stop milking her and dry her off) does she even begin to dry up in the fronts - so lactating for 10, 12 months without drying those quarters off, and I promise you we are unable to get milk from the front quarters. She's not milking out of them at all.
Given that, I'm not sure it would work to just dry off the back quarters on your nurse cow. As long as she's lactating - from the front quarters - she's going to have milk in the backs. Might make those back quarters hang lower anyhow; I know this cow I described above is almost lower in the front than the back, and her udder is already down by her hocks, if not lower. She milks really heavy.
Sorry for rambling so much...hope that makes sense and helps you anyhow. :lol: