Setup the configuration using visual studio normal startup project configuration.
Add a name in the "Name" text box.
Save current - will save a new configuration with the selected name. The configuration will hold the current startup project configuration of the solution.
Configure using startup manager:
Press the "Edit Current" button.
A window with all the projects listed in a grid will appear.
Check the projects you want to be set as startup.
Fill the "Configuration Name" text box with a name for the configuration.
Press the "Save" button to save the configuration.