There are a few ways to accomplish this.
Option 1: /etc/fstab entries in each distro
Updated, answer with the method I've personally been using for some time.
In each distribution, run the following command, one-time:
echo "/ /mnt/wsl/instances/$WSL_DISTRO_NAME none defaults,bind,X-mount.mkdir 0 0" | sudo tee -a /etc/fstab
1 (Credit and thanks to @mtraceur for the comment/suggestion and subsequent edited command. Simplified quoting is always a welcome change!)
After terminating the WSL distribution and restarting (to process /etc/fstab), your distribution will be available under /mnt/wsl/instances/<distroname> from all other WSL2 distributions.
2 See Option 1.5 if this doesn't work
See my related answer on Ask Ubuntu for details on why this works.
Option 1.5: /etc/fstab workaround for recent WSL releases
Under recent WSL releases (0.47.1 and later, I believe), a timing issue/change means that the fstab is processed and mounted before /mnt/wsl (a tmpfs filesystem) is mounted. If you are using a recent WSL release, and you find that /mnt/wsl/ does not contain an instances subdirectory, then:
sudo -e /etc/wsl.conf
And add the following:
[automount]
mountFsTab = false
[boot]
command = sleep 5; mount -a
This should wait long enough before processing /etc/fstab to allow /mnt/wsl to be mounted by WSL first.
If you are using Systemd, you can also create a .mount file to workaround the issue. See @SteveMeierhofer's answer for details.
Option 2: Use wsl.exe's stdin/stdout
(Bonus #1: This method works on WSL1 as well)
(Bonus #2: It requires no advance configuration)
(Bonus #3: It will work even if the second distribution isn't running yet. Option 1 requires that the distro be running in advance for it to be available in /mnt/wsl.)
For certain use cases, you can just use normal input/output redirection through the wsl.exe command to access a given file. For instance:
wsl.exe -d otherinstance cat ~/myfile | diff myfile -
or
diff myfile <(wsl.exe -d otherinstance cat ~/myfile) # bash (probably all posix)
diff myfile (wsl.exe -d otherinstance cat ~/myfile | psub) # fish
This is similar to how you might perform the same operations on a remote host through ssh. You can even send entire directories through the pipeline using tar (or other means).
This also does not require the second distro to be running in advance
Option 3: ssh
For the rare case, you can even set up ssh servers in each instance. The only case I've come across where I have to do this is Ansible, although rsync might be a possibility as well.
Note that setting up ssh on WSL instances isn't all that straightforward, but it's probably easier if you are only accessing it via localhost. At the least, you'll need a separate port number for each instance. I recommend reserving 22 for the Windows host itself (potentially using the Windows OpenSSH Server).
Option 4: A bind mount in /mnt/wsl
Older version of Option 1, left here for posterity.
Adapted from this GitHub comment, you should be able to do:
mkdir /mnt/wsl/otherinstance
wsl.exe -d otherinstance -u root mount --bind / /mnt/wsl/otherinstance/
Honestly, this one scares me a little bit, because the "otherinstance" shuts down soon after the command is run (unless it was already running elsewhere). However, the mount seems stable, probably because (as later comments in that GitHub issue mention) all of the "drives" are available from the WSL2/Hyper-V subsystem anyway; they aren't necessarily dependent on the instance itself.
Regardless, it would be easy to allay my (probably unfounded) fears by simply running the "otherinstance" manually and performing the mount --bind from that instance itself.
