One limitation of Unity's constraint system was the lack of multiple offsets for Rotation and Position Constraints. Meanwhile, Parent Constraints can have multiple offsets.
I noticed that you followed Unity's template and only Parent Constraints have offsets for each Source.
I would like to recommend that the new VRC Rotation and Position Constraints are allowed multiple offsets as well.
The work around for this particular issue is to add a copy of your Target object inside of each Source, then constrain to those duplicates with a 0 offset.
Once again, thank you so much for your work in getting these systems implemented!