Were you able to determine the identification of these two birds - or rather, shorebirds, from this post?
Are they the same species? The answer to that is a definite no. With that in mind, any clue on what one of them may be? Or is that perhaps even more confusing? This photo in particular makes it more difficult because you cannot see their bills as they are tucked away for a little nap.
However, we can actually still accurately identify these birds. The bird on the left is a very common shorebird migrant while the one on the right is a rarity. The left bird has a uniform gray-ish wash with black and dark wing coverts. It is more compact and round with a slightly smaller head than the thinner and lankier bird on the right. The left bird is a Semipalmated Sandpiper. Coloration can be a tricky thing on a computer monitor since it can vary, but for the individual on the right you should be able to see extensive rufous coloration on the back of the bird. This is on its scapulars, and if you check out the wing coverts you should be able to see they are much more gray, especially nearing this rufous area. This is a Western Sandpiper, and a bird more extensively discussed here in this entry on the Audubon Alliance for Coastal Waterbirds blog. You will be able to see more photos there including the hugeeeeee bill.
I would suggest looking at them in overall terms, not specific feathers, and you can get a good sense of the general difference. Sometimes keeping a wider and softer eye rather than trying to examine every single speck of a bird helps you see the more subtle differences in a much more clear way.
Photo © Scott Kruitbosch and not to be reproduced without explicit permission