According to Application Tunnels public documentation:

Each self-managed instance can receive only one tunnel from admin.atlassian.com. This limitation will affect you if you’d like to link multiple cloud organizations to a single self-managed instance, which isn’t supported.

https://support.atlassian.com/organization-administration/docs/what-are-application-tunnels/#Limitations

While it is good from security perspective, there should be a feature that would allow admins to configure more than one organisation they own with their own on-prem instances. This would help with partial migrations, when still some data needs to stay on-prem but that data needs to be available for more organisations (for example on BB DC linked from multiple instances in Atlassian Cloud in different orgs own by company)