Wow, what a striking image!

I love the composition - the way you combined the two separate images is just wonderful - and the effect you've achieved with the chalk, you managed to preserve the depth of both images.

As for the critique - there are a few anatomical misinterpertations I'm sure you've noticed so I won't point them out - just for future reference (because let's be realistic, it'd be rather hard to correct a traditional image) - the top horse's nuzzle is a bit long - or it just seems long because that ring where is mouth is has been drawn a bit small compared to the reference image.

What I can say that will matter for this drawing is I think it would greatly benefit from a background - even a simple one.
I was looking through my faves to find you a tutorial about a simple background traditionally but I can't find it. I'm sure you know how to make one though - it seems like you started to on the left of the top horse - I'd just like to see it encompass the entire painting, I think it'd give it more depth.

Another thing is - if you notice on the bottom horse in the stock image there's a little bit of the edge of the tail between is hind legs, which really adds to the image.
I personally believe in adapting the reference to your needs and not caving to the way it looks on its own. Concerning your drawing - I believe that if you add a bit of mane down his back (edges of hairs) and a bit of a tail - either between his legs or otherwise it would truly benefit your drawing, because it would add more depth to it. I'm not talking about a full mane and tail - just little tiny hairs flowing.

Anyway - all that said, I loved this image, it's very well done