These two lines are basically the center of each of the transmitters, so even if we have two transmitters right now doing the swipe, it still looks like one wave.
So long as the lines aren't parallel and they intersect over land, there'll always be a position where the two lines converge from which you could see exactly two trees.
For example, if you heat table salt you'll always see the same two lines in the exact same part of the spectrum as certain electrons in sodium absorb energy and then return to their ground state, giving off light.
And when you have high levels of HCG, as it's filtering through that stick, if there's a positive result, you'll see two lines because the HCG will bind to one of the lines.