I’d rather work in the same project.
But if you wish to work with subproject.
- Create the arrangement skeleton first.
- Create region for each part. With is time signature, tempo map, etc.
- Write at least one instrument, perhaps rhythm section for each part. Thus you’ll know if the arrangement could be stitched seamlessly.
- Create subproject for each part. Work the details arrangement for each individual parts, from its own project.