How to use 1 Data Volume with 2 MacOS versions?

Question

I am running MacOS Catalina installed on internal SSD which creates two volumes:

1. Macintosh HD
2. Macintosh HD - Data

I am reluctant about moving to Big Sur, so wanted to test it out first. I created a new APFS volume named "Big Sur" and installed Big Sur on it. Now, when I boot to Big Sur, it is storing data in the new volume "Big Sur - Data"

Is there any way to make it such that both Catalina and Big Sur use the same "Data" volume?

Answer 1

Have a look here.

Basically, the answer is no, unfortunately. It's a matter of firmlinks between the data and OS volumes, and relinking those would cause issues:

No, the firmlinks are bidirectional so you should have two pairs of filesystem if you need separate systems. The good news is that with space sharing and the smaller size of the system, having two Catalina around is ~22 GB for my Macs.