I don't think I'm going to tell my potential employer that I have another job.
Is there a reason you do not want to tell them you got another job?
I don't see why I should have to tell them.
You should just tell them the situation. You should not let down someone who is counting on you.
I like when people don't know I have another job.
It's reasonable to let your employer know that you have a second job so that they know what they can expect from you. They may be hiring you on the basis that you'll be able to do certain things that you can't do because of your second job. If you end up not being able to meet their expectations, you may let them down and lose your job.