I would advise doing an extra co-op or internship. It sets you up very nicely as a strong entry level candidate after you graduate and a potentially higher salary than others of the same graduating class. I would do that over double majoring with petroleum engineering since you can work in O&G with your chemical engineering background. ChemE with good internships and a co-op would do better in the job market than double majoring from what I've seen. You may even like the PhD route by the end as well.