What DC says, mixes get turned down all the time, often impacting our schedules for the worse. For myself I'd say the rule is that mixes won't be approved before the mastering date, and the the refreshing exception is getting mixes a day or two before. Even with non-label clients musicians these days are pretty picky about what goes out the door, especially since they know that in this era, everything can be fixed, overdubbed, tuned etc. they're always striving for the last inch.
Back to the topic, can't say I've ever felt the need, but something like this...
Drawmer Three-Sum Multi Band Splitter | 3sum 3 Sum | Soundpure.com
I think there's other similar options out there, or you can always ask a tech what it'd cost to put one together if you have certain specs you need.