Suggestion 1:
The linkage of projects depends on the purpose and business scenario.
However, few of the linkages are:
1. When you are settling one project to another, the linkage is the settlement rule itself.
2. When you are working between two different systems (two different production servers), a project coupling scenario is another linkage example. An Order recieved in one system (where a project is created initially), and transfers (outsources) this work to another Company code/Plant (which runs on another system) and because of this another project is created.
This transfer happens through an interface. In such cases, a user defined field can be introduced in the Project which mentions the partner project number.
3. The scenario mentioned in point 2, can also be implemented
in the same system.
etc....
Suggestion 2:
As per my Knowledge, we can link multiple projects with the below two functionalities.
1. Requirements Grouping Functionality.
2. Giving assignment of WBS Element to an Activity in another project.
1. Requirements Grouping Functionality :-
You can group different WBS Element pertaining to multiple proejcts for Material requirement using T-Codes GRM3/GRM4.
2. Giving assignment of WBS Element to an Activity in another project :-
By assigning one WBS Element to Network activity of different Project, you can link both projects for scheduling point of view.
You can manage the dependency of executing the successive activities by giving relationship with that activity. This is little bit complicated to check the changes after rescheduling any of the project.
or
Make use of User defined fields to link the projects together.
