Doc advised me to take metformin twice daily. I feel nauseated and tired so I am taking oly one. Is that ok?

Talk your doctor. You are having side effects of metformin discuss with provider about reducing dose or discontinuation.
Listen to doc. Your doc adjusted your dose of medication to the lab results he/she obtained. You need to stay with the dose or visit the doc again and discuss it with him/her. Diet and exercise can help lessen the dose but your doc needs to know. Peace and good health.