Mike,
When you open MyEclispe workspace, all the projects that are in the workspace will be listed under Package explorer by default. If you right click on any project and click “Open in new window”, only that project you selected will be opened in new window. You can goto rest of the projects by clicking on “Up to workspace button” on the top of package explorer in the new window.
It is not possible to show all the projects in the package explorer when you access “Open in new window” option.
However, this is a base eclipse feature. I suggest you cross post on Eclipse forums as a feature request.