Is it ok to take pepcid 20 mg (in am) and nexium 20 mg (in pm) during same day for severe case of acid reflux?

Why do you take both meds at small doses? For severe GERD, you should take maximal dose of Nexium, 40 mg, twice a day until symptom is under control then 40 mg daily then maintenance dose 20-40 mg/day. If you prefer Pepcid you can take 40 mg, daily or 20 mg, twice a day then maintenance dose 20 mg, daily. You should only use combination if either fails to control symptoms at maximal dose.

Yes, if so prescribed. Different mechanisms of action, combination sometimes used with clear goals and for limited amounts of time.