Add support for displaying the UI when multiple "to" post types are defiend.

[gthayer]

Is your enhancement related to a problem? Please describe.

The plugin supports the ability to define a "one to many" relationship, however it also intentionally disables the metabox from displaying within the editor. I am unclear why this was disabled originally, but it seems the architecture does not easily support the feature because of how relationship keys are detected and constructed.

Designs

No response

Describe alternatives you've considered

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[gthayer]

I have a PR for this issue which I think is ready for review.

cc @jeffpaul