Create a Root CA with Subordinate CA already existing

I have to provide a Subordinate CA in one location with a Subordinate CA already existing in another location (same Domain) but no (Offline) Root CA has been installed.

I figured to have 2 Subordinate CAs you must have a Root CA. Also that a Root CA has to be installed first before installing Subordinate CAs.

Question is how to go from here to provide a second Subordinate CA. If i delete the existing Subordinate CA to install a Root CA and then create the desired Subordinate CAs i risk major problems with existing certificates i assume.

Thanks in advance!

Alexander