Is it possible to use multithreading in multicore systems? What is the certification impact?

Simultaneous multithreading is out of scope of A(M)C 20-193 as this is not specifically a multicore issue.

In practice, multithreading adds even more complexity to the testing and reasoning about the complex behaviors of a multicore system. As there is no additional guidance or means of compliance available from A(M)C 20-193, any approaches to making a certification argument should be discussed with your certification authority.