Can replens be used in conjunction with femring?

Yes, but alternate: Replens is designed to replenish vaginal moisture over time and works differently then Femring which replaces the estrogen of the vaginal lining. If you are using Replens for one week or more, then stop it when the Femring is put inside the vagina. I applaud you for trying to do something for "dry" or painful intercourse rather than cessation of sexual activity.