how handle home folders with dual-boot (Catalina and BigSur)?

I am happily running Catalina (on a Macbook Air 2017) and am tempted to upgrade to BigSur, but I'm a little leery of breaking apps, particularly Arduino IDE (which appears problematic, from the little searching I've done). So I want to make the system dual-boot - my existing Catalina installation, and BigSur (or perhaps Monterey) - and live with the new OS for awhile before I commit.

There are numerous articles describing the process, in brief: use Disk Utility to add a new volume, download the appropriate installer, and install the new OS on the new volume. But beyond that ?

It seems there are two ways to go:

1. Use Migration Assistant to import everything (apps, home folders, other files) to the new BigSur volume, thus two completely independent systems.

2. Use SystemPrefs -> Users&Groups "advanced" to point the home folder (when logged into BigSur) to the Catalina volume.

It seems each path has advantages and disadvantages and I'm not sure which to do. #1 has the disadvantage that there's two copies of user data, thus the issue of keeping in sync - probably best achieved by only modifying files in one. But #2 has the disadvantage that certain files (e.g. ~user/Library) may need to be different for the new OS (I'm pretty sure that's the case for Arduino IDE).

How should I slice this cake ?

As requested below, here is the current state of my disk:

% diskutil list internal
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *240.0 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         239.8 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +239.8 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume MacHD - Data            88.3 GB    disk1s1
   2:                APFS Volume Preboot                 91.1 MB    disk1s2
   3:                APFS Volume Recovery                529.0 MB   disk1s3
   4:                APFS Volume VM                      4.3 GB     disk1s4
   5:                APFS Volume MacHD                   11.2 GB    disk1s5

After installing Big Sur ...

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *240.0 GB   disk0
   1:                        EFI ⁨EFI⁩                     209.7 MB   disk0s1
   2:                 Apple_APFS ⁨Container disk1⁩         120.0 GB   disk0s2
   3:                 Apple_APFS ⁨Container disk2⁩         119.8 GB   disk0s3

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +120.0 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume ⁨MacHD - Data⁩            84.9 GB    disk1s1
   2:                APFS Volume ⁨Preboot⁩                 91.1 MB    disk1s2
   3:                APFS Volume ⁨Recovery⁩                529.0 MB   disk1s3
   4:                APFS Volume ⁨VM⁩                      1.1 GB     disk1s4
   5:                APFS Volume ⁨MacHD⁩                   11.2 GB    disk1s5

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +119.8 GB   disk2
                                 Physical Store disk0s3
   1:                APFS Volume ⁨BigSur⁩                  67.3 GB    disk2s1
   2:                APFS Volume ⁨Preboot⁩                 287.6 MB   disk2s2
   3:                APFS Volume ⁨Recovery⁩                625.7 MB   disk2s3
   4:                APFS Volume ⁨VM⁩                      1.1 MB     disk2s4
   5:                APFS Volume ⁨Big Sur⁩                 15.3 GB    disk2s5
   6:              APFS Snapshot ⁨com.apple.os.update-...⁩ 15.3 GB    disk2s5s1

Solution 1:

Personally I advise you to separate the 2 systems.

Indeed if you access Catalina data from a higher system, you run the risk of not being able to access it from Catalina, the compatibility between systems is always upward and rarely downward.

Then I advise you to create a new container for BS or Monterey so as not to mix up the systems.

When you have tested the new system and if everything is ok, it's easy to clone the new system in place of the old one and free up unused disk space.