What about a Merged List?
As an outsider, the problem I see is this may be like having your ex say "we can both use my attorney for the divorce, don't worry he will treat you fairly".
Exactly how is ALPA going to best represent the SW pilot group for seniority integration, please look at other airlines starting with Flying Tigers to AA/TWA, to see just how ugly this gets.
The best contract I've seen for a similiarly sized airline (a little outside the box) is Net Jets. My point is competition is good, even for unions, they need to be forced to compete for your 2%, and once they get it they need to continue to earn it.
My biggest complaint with ALPA is that for years they have not paid real attention to the regionals, and now that is where much of the mainline narrowbody flying is at half the pay rates. And don't just accept the "union is you" response, look at Delta vs. ComAir vs. ASA, it's all ALPA. I'm not smart enough to know what the fix is, but I don't think ALPA has figured it out either, and a little competition and embarassment might bring some focus to this area of the industry.
I will go back to my orginal point exactly how are you going to be best represented?