There should be a way to display two project calendars at the same time, or two share a milestone between two or more projects

There are plenty of situation when you want important milestones to be known by all projects manager, to avoid milestone collision, especially when all the project are hosted on the same server (for instance).

So the best way would be to be able to create a milestone visible across all projects (It could be for instance "The server is down for maintenance that day, so do no plan to release some newsletter that same day")

Another option (less practical, but still possible) is to allow project managers to display two project calendars at the same time. That way one can define one central project that the project manager should look at at the same time he is organising its one milestones, and he can add some of its more important milestones into the project calendar.