Let's say if a contract role allowed work from home. Is it possible to work with 2 clients at the same time? Say you had a full-time activity and contracted as well?
It's perfectly acceptable to have multiple clients in the case where you are contracted as a consultant or sole trader unless specifically contracted otherwise.
If you are a full time employee, then that's another story, in that case you need to check your contract very carefully because you can get into quite a bit of trouble if you're non compliant. But if you're working from home and having no issues with your work, providing your contract doesn't stop you, many people take on side jobs. I see nothing unethical about it so long as you are professional and keep the two totally separate.
If your employment contract does not prohibit it, then it comes down to the laws in your jurisdiction, local culture and any other restrictions.
For example, licensed professionals may be required to adhere to a code of ethics that prohibits such 'moonlighting' activities without written knowledge of all parties, even if their personal ethical standards and laws applicable to the general workforce would permit it.