How to paint 2 different colors with a knockdown finish?
Painters secret, purposely withheld.
Several reasons. One is that the tape would have to go on the ceiling, often the walls are knockdown and ceilings are popcorn. The tape would pull material off the ceiling.
Second, it's tricky getting two coats of paint on the tape and then pull it without peeling the paint off the wall. Normally, you would use a razor blade, but at the ceiling/wall corner, it will leave a black line.
And finally, on textured corners like that, it's better to freehand a straight line by eye. Slick mentioned the lumpiness of texture, and using tape will get a straight line, but it won't look straight because of the lumpiness. Without seeing the work, it's hard to recommend or discount this method. If you do use the tape method, the tape has to be pressed firmly, 100% around all the little lumps and bumps. Paint the ceiling color on the wall and then the wall color right away, then pulled right away.