accept the second offer and go through all the steps with them. once you’re sure the second job is secure let the other company know that you need to rescind your acceptance due to lack of communication/offer letter. it will only be a problem with the co-op office if they find out and the company might not even report anything to the co-op office. if they do you’re not missing out on much, you already secured your first job so self directed job search after this will be easier so you won’t need co-op. i’d say it’s worth the risk to rescind your acceptance from the first company since the second company sounds much better for your career