You'd have to query the show sessions API with a different user to get the active sessions, then iterate over each one to see if there are any sessions by the user in question.
Seriously, though, if you're running R80 or R80.10, you should really upgrade.
If you're running R80.20+, assuming the scripts are modifying different objects/rules, you should have no trouble running the scripts concurrently.
The only command that is not supported in parallel is install policy.
You should be error-checking for policy install failures.
If it's because another policy is already installing, you should be able to determine this by reviewing the error you get and/or reviewing the running tasks.
That said, I'm not sure what you are doing is the correct approach.
Are all of these policy installs to the same gateway or different ones?
And keep in mind that policy installs can be disruptive to running traffic depending on a number of factors, so you want to keep plicy pushes to a minimum.