The primary gets first shot. They will be sent an offer letter. If they sign it, they will then be responsible for the Medical, Dental and Drug screen procedures. If they pass they will be sent tickets and will get the job. At any point along the way, problems can jump up. First of all they may decline the original offer, next they may flunk one of the PQ or drug exams. Finally, they may wait until the week before flying down and drop out.
As an Alternate you should sign the offer letter and get PQ’d medically as if you were a primary. Then you wait. If for any reason the primary drops out, the alternates are next in line for the job.
I don’t know the procedure for more than one alternate position. I didn’t think you could get two, but I guess I am wrong.