You have to call your mail order company and find out for sure because they're all different...some will allow an overlap, others won't. I think MOST of them will allow at least a week to account for shipping and you can send them a post-dated Rx. I work at a retail pharmacy and we can only allow a 3-day overlap with most insurances.
With 90 day scripts I can refill after 2 months on my present insurance. I do this with Ativan and Lunesta. Schedule 2 meds may be different.