Using just SSMS and SSMS only, you can't run them at the same time. You can run them from two different windows, but there will be lag while you click between the windows. If you must start them at exactly the same time, you can use Agent as was suggested, or you can look into threading the execution in PowerShell (or a programming language). Then you can get simultaneous execution.
Why are you trying to execute them at the same time?
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Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning