As concordnz suggests, the overlapping arm won't work as intended if you don't have curtains that hang in pleats. Many blackout curtains are pulled flat so there is no light-leakage at the bottom and top. But then there is no pleat to block light leaking through the overlap.
That's why allowing a pleat to form at the overlapping arm will that the front curtain touches the back curtain to block the light. But it will look a bit strange just having a pleat at the overlap.
Another option, which I've seen before, is to have zip attached to join the two curtains. This could be an option if the curtains can't be extended to use the overlap arm. The zip would not be visible because it would sit behind the leading edge just like on my raincoat.
If my wife were here she could probably tell me the technical names for all this technology.
