Tie one request to two different projects

As far as resolvable objects go you can only have a 1:1 ratio --- 1 request = 1 project.

You have the option to do cross project predecessors that could tie multiple projects together if certain tasks are somehow related to one another. This could work to have 1 project resolve the issue but dependencies from another project in order to complete the project that resolves the issue.

Hope that makes sense. 

As far as I'm aware, when Workfront resolves a request by being converted to a project - going back and editing that request would not likewise update the projects.  When those requests were converted into a project the data from the request was used to create the project, after this happens there is no way for any info added to the original request to flow into the (now) project.

These updates will need to be done in the corresponding projects - the ability exists within the project to go back into the custom form and provide updates.  I would just warn you that updating the forms in this manner does not notify anyone in the project, so if important project details were updated you would be wise to communicate these manually.