Create a directory on the host and add a mountpoint to that directory to both containers. Make sure to use the same user/group ID in both containers and allow read/write access for those user/group ID (+100000 when unprivileged) on the host.
Create a directory on the host and add a mountpoint to that directory to both containers. Make sure to use the same user/group ID in both containers and allow read/write access for those user/group ID (+100000 when unprivileged) on the host.