RegularJoe,
I was under the impression that the specialist testing day is the same for ATCO as it is ADF Pilots, the only difference being the pass/fail mark for the tests. Ie. the first time I went (I was unsuccessful this time) there was an ATCO guy going for that position and was in the tests with the Pilot applicants like myself. I failed the gauge reading test for Pilot but was notified afterwards that I was still eligible for ATCO based on my scores if I wanted to pursue it.