Possibility of creating a cross-project milestone

When a project is composed by several subprojects (for example a server and a client project) each subproject has his gitlab (project) page so that it can track correctly its issues and development, but we lose the possibility to have a milestone that includes tickets from both projects.

For example we would like to define the "v1 release" that needs to have certain features developed both on server and client, but how to have the list of tickets for both projectes on the same page?