When doing two tone paints, the easiest way to make straight lines without having the big edge or ruining the surface would be to lay your base layer down and then when your ready for your second color, let the first color tack off for at least a half hour or longer, this way it gets nice and hard and you dont have to worry about it peeling off when you pull the tape off. To tape of the second color First start by taping the last line(where you want your last line to be). Then start up a little higher then where you are going to end usually like 1/16th of an inch or so and use fine line tape. Then lay another line of fine line tape at 1/8th of an inch or so. Spray, let tack, peel that line of fine line off, spray, pull the second line off, spray, then pull of your last line and clear you should have no lines and should not ruin your base color. Hope this helps you out.
Jesse