I loved it! so short but it spoke volumes... Truly heartachingly beautiful.

The feeling that this brings is truly heartbreaking. She's not there, so he must wait forever.

"Chills tapped on his spine
as the street light flickered."

I love the imagery of this, it's a unique way of saying 'shivers run down one's back'; they way you worded it makes it more unsettling, personifying the shivers. The contrast between that and the flickering street lights paints this to be a dark/horror piece. You also wrote it with pace in mind: direct to the point with no flowery language to offer a reprieve.

"He glanced over his shoulder
and saw her shadow
linking arms with his."

Keeping in track with what I said earlier about pace, you continue that, offering a fact to the narrative and letting suspense build. Shadows linking with one another is worrying, keeping in line with the dark/horror tone you've set earlier.

".
.
.
The night deepened;

he waited for her."

And all that suspense you've built pays off; the reader comes to learn that he waits for her/the shadow. I really like the usage of 'night deepened', it's vivid and extremely fitting to this poem. It ties back in with the shadows linking with one another, painting a picture of more and more shadows coming out (?) or perhaps a lack of light (symbolizing hope, etc). The decision to wait for her is intriguing, is he giving up all hope? or is he so distraught by a loss (her shadow) he's willing to do anything to get her back.

A really nice write that's sort of a departure from what you normally write. I really wish I had a nomination :(