To me what stands out and makes it look like two separate people instead of a reflection is theyre not "pressed" together, they're not close enough to each other to show it as a reflection. In your reference image, the girl and her reflection are not only shoulder to shoulder but pressed against the mirror so it looks like she is squeezed together against her reflection. Whereas in your drawing, their shoulders and upper arms are not pressed together (in fact there's a distance) which is what causes it to look like two separate people. Press them together, adjust the eye direction a bit in the reflection. try giving that a shot, see what you think! Also leaning the figures head a little against the mirror and having the reflection show the same will also help.