How do you feel about employees moonlighting (having a second+ job)?

Depends on what the 2nd job is: is there a conflict of interest and does it affect their work with us 

There is a saying in Mexico that says “Those who have 2 bosses, it will let down one of them”.

Wouldn’t that depend on whether or not they were allowed (or disallowed) by company policy and/or employment agreements; and/or whether or not they were fully transparent vs deceptively holding two jobs at once? I think you should clarify your question to include this context.