It's stacked. If a script fires while another is already running, it'll wait for completion of the first.
At least that's how it was the last time I checked. Someone else may have different information (if it's changed).
As plugin vendor we were informed some time ago that scripts on server may run in parallel with multithreading and our plugins should be aware of that and not crash.
So it’s changed. Thanks, Christian.
You may need to test and see what happens.
e.g. run a scheduled script and run perform on script with two scripts where you log beginning and end. The scripts could do a loop for some time to run a few minutes.
Than you would see if they run parallel.