Legal means no contravention of law

There are 3 aspects of law. Employment, tax and medical registration.

Employment
If you are a Singapore or PR, it is perfectly legal for you to work at any workplace.
Foreigner can only work at location stated in their employment pass.

Taxes
If you locum, the income you earn you declare and pay taxes, it is legal

Medical registration act
If you are FULL registration, u can locum anywhere


Now the question is does your company HR has policy to stop you from working 80 hours or locum at alternative workplace?

Again this is ur company internal policy.
Nothing to do with law.

U may be fined or go to jail for not paying taxes, working with valid workpass or practicing without full registration. But u the state isn't going to care if u locum or not