You said "both fast track lines" which makes me think you're not talking about fast track at all. There is only one entrance to Fast Track, and it's between both immigration halls. It has a big archway, nearby the visa on arrival desk. I don't believe PR is one of the listed qualifications for entry for Fast Track. The area that I think you are talking about is the "priority" lane, which is next to the Thai citizen lane. There would be 2 of these, because there are 2 immigration halls. Each one has their own priority line, which is normally for business class and disabled people.