Don't know where ya'll got this old worn out rumor all fired up again, but it's been passed around by everybody's neighbor's aunt, who's brother-in-law's daughter's college roomate is an FA at Spirit, since the original AA / US Air merger. But yeah, it would have to happen... Eventually...
As for if / when it happens; relative seniority integration is a B!tch! ALPA merger by-laws state that if 2 ALPA carriers merge, the integrated seniority list shall be by DOH, IIRC.
However, when Pinnacle, Mesaba, and Colgan merged to form Endeavor; relative seniority ended up playing a big part. It really sucked for an 8 year guy to be slinging gear for a 2 yr CA. A lot of heartburn there...