Some employers have contracts that require that you work for them exclusively. That is especially true in field sales, where people who represent multiple product lines (manufacturers agents, distributors, dealers) are quite different from the company who hires an exclusive field salesman.

If you do not have an exclusivity or non-compete agreement that conflicts, moonlighting is usually allowed. Most employers like to be informed. And obviously it cannot be with a competitor. But if you can handle both jobs, why not try it until conflicts arise.

Of course when either employer is advised of the other, you risk getting fired. But you could always ask each for permission up front. Sometimes they will easily agree.

Getting permission for outside consulting has not been a problem where I worked.