As the achievement states, you need to guide any of the 3 grand companies to 200 victories COMBINED. I have multiple victories across 2 grand companies, yet the Behind Enemy Lines achievement is only counting victories for one grand company (the one I'm currently enlisted in).

Exhibit A
http://puu.sh/eSzcr/d925507f30.jpg

Exhibit B
http://puu.sh/eSzeX/a4e9a9a0be.jpg

Exhibit C
http://puu.sh/eSzgL/4001c68ca7.jpg

As you can see they're not cumulative as the achievement in and of itself states.