To open multiple projects in a single session, use any of the methods described in Opening a Project to open an additional project while a project is already open. The Vivado IDE prompts you to close the current project. If you do not close the first project, both projects are opened. Each open project has a separate IDE window.
When opening multiple projects from the same Vivado IDE application process, be aware that the commands used in all open projects are written to the Tcl Console. When reviewing the transcript of commands, it might not be clear which project the commands are associated with. In addition, there is only a single vivado.jou and a single vivado.log file for the application for all projects.