It`s not a merger, but rather a close-cooperation.
Pilot`s seniority is not an issue, as the lists are not integrated.
There are some issues, however over code-shares. Concerns over Northwest doing AMS-Bombay operations (politically driven). Concerns over Northwest doing codeshares with Transavia(=80% KLM) on certain inter-European routes.
But nothing major, in general there is good contact between both pilot`s unions as well.