You dont need to keep the high and low uvs synced up for any reason, generally. When you bake down it will transfer all of the info to your low's uv map.
For hair "cards" i would suggest using a tiling texture/a section of your uvs that you instance for this purpose. It doesnt really need to have normals info persay even. This is probably the simplest thing you can do, i'm sure there are some more advanced solutions but i would stick with this for now.
If you've got some images to show what you've got so far, and/or some references to show what you're trying to accomplish, it would help out a ton.