Hey ergin,
There are several ways you can run multiple tests at once. I'll describe two main scenarios and we can provide more detail if needed depending on which one you want.
1) Testing Multiple UUTs at Once
If you want to run the same set of steps on two units at the same time, you can use either the Parallel or Batch process models to handle the multithreading. To change the process model to one of these, simply go to Edit > Sequence File Properties, click on the Advanced tab, and set the Model Option to ParallelModel.seq or BatchModel.seq.
2) Running Multiple Tests on One UUT at Once
If you want to speed up your testing of a single UUT by running multiple tests at the same time,you can put one set of tests in a new subsequence and call that subsequence from your main sequenece with a Sequence Call step. The Sequence Call step has a setting to execute in a new thread, and if you set that, the subsequence will execute while the rest of your code continues to execute. This will allow you to run multiple tests at the same time.
Does one of these options sound like what you are looking for? If so, we would be happy to answer any further questions you may have about it. If not, just let us know what you want to do, and we can probably suggest a way to organize it in TestStand.