Actually quite do-able. It is done under something called MOCT. Multiple Operating Certificate Training. There is nothing in 121 or 119 that precludes a pilot group being trained to operate two certificates. It only involves having the ops specs, FOMS, and training manuals similar enough. Each certificate has to meet the requirements of pt. 119 and have the systems and manuals in place, but nothing keeps them from having the same operating manual, ops specs, training etc. The PRIA and Drug testing comment is BS as both or either can be accepted by both certificates. You are not being told the whole story. Call Chris and ask him. He knows.
It's kind of funny how you "know" all of this. I've been through this process in a previous life, have you? I don't need to call anyone here, because I was involved with this previously. If all you said was correct, how come it hasn't been set up for 5Y guys can't hop out of an Atlas Bird and then fly a GSS bird immediately after? That'd add 3 airplanes and a ton of efficiency, right?
While you are absolutely correct about they can have the "same" manuals, etc. in place. How come it's been over seven years and they don't have the paperwork all ironed out yet? If they did that, it would show in good faith that there was to be a merger.