There is a git command called git-quiltimport. Its man pages says it "Applies a quilt patchset onto the current git branch, preserving the patch boundaries, patch order, and patch descriptions present in the quilt patchset."
However, it doesn't define what a "quilt patchset" is. What is a "quilt patchset"?
Quilt is a patch managing tool. The idea is that it makes it easier for you to maintain multiple, discrete sets of changes in a single source client, even if they touch the same files.
The official site is sparse on details, but worth if you are thinking of using it.
If you didn't use quilt, you would probably just have to create another client to maintain the separate change sets, but you would have to patch back and forth changes that would benefit both client source trees.