It is possible. (AFAIK it is not possible to open a workspace editor and select "run all projects")
Possible ways:
1. Open a tab for each project/testSuite in your workspace and run the testSuites simultaneously.
2. Use the testRunner.bat/sh to run an individual project, and create multiple batch/shell scripts, one for each project. (Or use a CI (Continous Integration) tool to do this for you, also helpful with regression tests)
3. Open an instance of soapUi for each project.
4. (Requires a bit of scripting knowledge (groovy)):
Or you can write a groovy script that will run each project in an individual thread.