With PanAm, we had a pilot seniority list, with the ALPA union.
However professional flight engineers (PFE) had their own list, with FEIA union.
After 1963, newly hired F/Es were generally pilots (a CPL/IR was required) and were on the pilot seniority list. but PFE were non-pilots, with F/E licence.
Like you, I've been a member of both unions.
Actually, at Pan Am, PFE originally referred to a Pilot/Flight Engineer. See Ed Spellacy's note here:
http://www.galaxycouncil.com/Panamus...red_Minkle.pdf
I agree that at most other places a PFE is a non-pilot flight engineer.