For US universities, is the post-study work permit duration two years if the course is STEM?

No, for STEM courses in the US, the post-study work permit is not limited to two years.

You receive an initial 12-month Optional Practical Training (OPT) period after graduation. If your degree is STEM-eligible, you can apply for a 24-month STEM OPT extension, allowing you to work for up to 36 months (3 years) in total after your STEM degree.

Your employer must be enrolled in E-Verify, and the job must relate to your field of study.