Is there anything in PA Employment Law that says I can't work for two companies at the same time?

I assume, since you are asking about employment law and not the interpretation of a contract, that you have no employment contract which prohibits you from "moonlighting". There is no general prohibition against working for more than one business, although it could get tricky, especially if they are competitors in any way.  You owe your employers a duty of loyalty, which you can't breach.  For example, you can't do work for one employer using the other's time or resources - you can't spend your time at one job doing work for the other, you can't use the postage meter from one job to mail correspondence for the other, etc.  Also, you can't use confidential information from one job on the other.  If one employer has a confidential list of potential customers, you can't use it on the other job.  Also, you can't divert an opportunity, like a customer inquiring about one employer's goods or services, you can't divert that opportunity to another employer or yourself. You should also realize that, again assuming no contract to the contrary, there is no prohibition against your employer firing you for moonlighting if he/she/it finds out.