Multiple Repository connection From Single RMA

You cannot do this. The option to "Enable access to other projects" in the RMA generation process requires that these other projects be in the same repository and it says this explicitly in the documentation for Developing Rule Maintenance Application on page 365-6. The only built in features to work between repositories would be Lifecycle Management where you can publish from one repository into another to move things from development into testing, or push something into production. Anything else would require you to build something yourself using the APIs.