If you want to do the back and forth, you have a few choices. You are going to use the consistent screenplay direction INTERCUT to show that you are going back and forth. If you want to just indicate that you are going to be cutting back and forth right at the beginning of the scene, you can just create an INTERCUT type scene by including this in the heading. Example:
INTERCUT – PETER’S OFFICE/DENISE’S BEDROOM
This will show that the scene is taking place in both locations. If you do this, then the director, producer, and editor have the choice as to when they want to cut back and forth. If you want to keep the power when screenwriting, you can just put the INTERCUT in between each line of dialogue. If you do this, you essentially put the INTERCUT and the location above each line.