Exactly so, but there's no point to the first method; you are still getting 1:1 credit for kills, just stringing a lot of them together in a line.Stacking missions from the same broker means that when one is accomplished, one must exit the area (change the instance) then one can return and do the next and the next...
Stacking exact missions from multiple broker's means, accomplishing one accomplish all exact mission simultaneously.
Thus stacking still works, but there's two different procedures to follow.
Using multiple factions, you are working missions in parallel, getting multiple kill credit per single kill and maximizing your earning potential.