I've got 2 work permits but can only take one job! What do I do?

You will run into problems, you can use the work visa to enter China, the immigration won't check it, however, it is impossible for you to get a work permit and residence permit within 30 days after entry into China because you refuse the job whose invitation letter you used to apply for Z visa. The another employer can't help you, you make the thing complex, if you use the invitation letter of the job you accept, everything is ok.