To run several tests at once you must first make sure that only one instance of webstress7.exe is running in one folder. That means that if you want to run several copies of Webserver Stress Tool at once you must make sure that each EXE runs in its own folder. Simply copy the \program files\webserver stress tool 7 folder as often as you need it.
Then create a CMD batch file like the following:
Start \folder1\webstress7.exe \folder1\1stszenario.ini
Start \folder2\webstress7.exe \folder2\2ndszenario.ini
Start \folder1\webstress7.exe \folder3\3rdszenario.ini