To reach an elite level, you have to earn EQM (Elite qualification miles)
on a single airline between January 1st and December 31st

Based on the fare class and airline and your FF program
you could earn somewhere between 0.5, 1 and 1.5 EQM
per miles you flew.

All miles have to be on a single airlines. You can credit miles
from any SkyTeam airlines to any skyteam airline.
But you have to request that at the time of booking or
check in.

Just keep one FF account primary and credit miles from
all airlines onto it when you fly.

If you are in US. I'd say keep NW as your primary.