...to find the best distance apart from each other. I would say 15 meters is too far... about 10' (around there) is usually the limit for most speakers - after that, the center image will start to fall apart.You will probably want to strike a balance between the widest soundstage possible and the least sidewall reflections.

Also, try to keep each speaker in an acoustically similar place... not always easy, I know.

Good luck...