How do you get dividing line straight when painting the corner where two colors meet?
For this kind of thing, you could use masking tape. The line will be just as straight as your tape is. When you do it with 2 persons (one putting the top in its place, and the other below), you can put on a long strip of tape in one time. Then you'll get the straightest line. The tape should be locked to the wall all-over (on the seam edge), otherwise the paint will creep under the tape. You sort of dab the brush on the tape, not wipe the paint under it.
Also important: remove the tape ASAP, otherwise it will get stuck too good, and take paint chips or more with it when you remove it.
Good luck with your job, Evelien