Getting vocal parts together (linking?)

I am rearranging a score from harp plus single vocal into piano with SSA vocal. This makes for a very long score, so I want to be able to pull out the piano and vocal parts separately, but have all the vocal parts together.

I added the two additional choir parts as new parts, but they should be linked, just like the right and left hands of the piano are linked. How do I set this up correctly? I'm working on the piano section now, so I want to get the vocals set up right before I dive into them.

Thank you! I use Musescore about once a year, so always relearning. Score attached in case that's helpful.