I think that captainv is right. If you are honest with the Atlas people, I imagine that they will understand. I doubt they expect you to hang out unemployed for an indefinite period of time waiting for them to call. Maybe you will burn the bridge with the LCC, but it is a bridge you don't want to recross anyway. All airlines know that this happens. It is part of the business. Nonetheless, I understand your concern. I hope it works out the way you want it!