Further to Dattatreya’s response be aware that if you link back and forth between multiple Project Groups you may have to schedule the Project Groups many times in the correct sequence to get the correct finish date for all Project Groups. I would strongly encourage you to merge them into the one Project Group to avoid this issue (you can then schedule the Project Group with the appropriate data date option set).

The only valid use I have had for Interproject relationships is where the planning unit is different. eg building project over exiting electrified rail line with building project planned in days and track possessions in hours (for track upgrades or working above high voltage lines prior to enclosure). All relationships go one way from the daily building project to the hourly possession works schedule.