Originally Posted by rcrw88
You could also work for both companies if you have the time? (unless its coop of course..)
Your profile says you're in engineering, if so your employment contract (the one you already signed) most likely states the job you accepted be your only employment (or something to that effect). This might be something worded like:
"Other work: During the Employee’s employment with the Employer, the
Employee shall devote his/her entire working time and attention to the performance of the Employee’s duties pursuant to this Agreement, and shall not, without the prior written consent of the Employer undertake any other business, occupation, work or employment, or become a shareholder, partner, director, officer or agent (except as a passive investor in a publicly traded corporation) of any other company, firm or individual without first requesting and obtaining the Company’s written consent."
Whatever you decide to do, please recognize that the engineering world is generally a small one. Chances are you'll come across people from Company A and B later in your career and you don't want to leave a seriously negative impression at either. That being said, going to an interview is totally fair game.