Why can't you make them two different assignment activities? The first with the submission and the group rubric and the second with the individual contribution rubric. Set the second to "no submission".
Put both grade items into a category in the gradebook and sum the two to equal 50. It's not perfect, but it would preserve that group grade since you don't need to override anything.
If you set the individual grade activity to visible groups, then graders can filter by groups as they grade it and have the group submission activity open in another browser window for reference. So, you'll get one grade for each group in the first assignment, then one grade per person in the second.