When I worked for Crossair in Europe we more or less merged two airlines - Crossair and Swissair to form SWISS International. The Swissair pilots did not want us to form one pilot group. But keeping it as two made both groups weaker.

The situation is not really the same, but having two pilot groups and seniority lists is not a good thing for the pilots. They will fight and snipe at each other and the management will simply sit back and watch gleefully.