It is a general rule of insurance that you cannot claim from more than one insurance policy at a time for the same event. This applies to a Medical Aid as well as a Health Insurance product.
You can legally own more than one health insurance policy but it would only make sense if they are designed to cover different things.
When you claim for a medical event, your claim needs to be recognised by (paid by) the first insurance policy you own.
When does it make sense to have multiple policies?
Short answer is when they cover different things. You would usually would have multiple benefits in a single policy but there are few cases when you might have multiple policies.
For example, your employer has a group scheme offering you a Day-to-Day plan and you take out your own Hospital Plan to get a more comprehensive suite of benefits. This is a common scenario and you are very much allowed to do this as both policies cover different events.