Learning from History.
It is worth looking back to the merger of BRAL & Brymon. Management have a positive interest in divide & rule and will grasp any thread of disunity and play it for all it is worth.
In the end, the possibility of legal action if they forced a merger that suited them, rather than let the union resolve it won out.
Trouble was avoided in that merger by hard work from both CCs and good common sense on both sides. The protective instincts of the troops in a senirority merger are music to management ears, so be careful and go for a merged seniority solution based on fairness not protection.
BALPA need to be given the hard word along these lines too, as whilst they are your best hope, they too are human enough to want a solution rather than war. Remember that everybody in these deals has an agenda!
Good luck, you'll all need it and besides, not everyone can walk.