Is it ok to take 2 tablets of Advil (ibuprofen) at once?

Yes. I assume 2tablets = 600mg. Yes it will be ok.
Yes but... If you have normal renal function and no gastrointestinal bleeding or ulcers, then an occasional dose of 400 mg Ibuprofen (2 tabs) may be ok. The maximum is 2400 mg per day. However, you should not take that on a regular basis - it will affect the kidneys and it greatly increases the risk of esophageal or stomach bleeding. Also, it depends on what other meds you take. Check with your doctor.