Government jobs or military career, you cannot hold secondary appointment/jobs.

For private sector, it depends. It goes two ways:

a. The secondary job employment is not a client of your primary employment. Therefore, no conflict of interest.

b. The secondary job employment is a client of your primary employment. So cannot. Conflict of interest.

Otherwise, no issue.

Your employer still has to pay for the time of your work commitment with them. If they hold their payroll due to your secondary appointment, please report to MOM. You must be paid accordingly to the work hours you committed with them.