I have two PhD offers. Can I accept one while waiting for the grades for my preferred option without the latter finding out?
You should contact both programs and openly describe the situation. 

Each individual academic field is fairly small, and academics do regularly talk to each other. Reneging on an offer will tarnish your reputation. Even if you successfully pull off this stunt, you will earn some enemies, which can bite you later (e.g. seeking jobs, applying for grants, etc).