Cute picture, the interaction in the poses is nicely portrayed. It already looks pretty good as it is to me. The only things I really noticed are the belly button on the guy in front is a bit too high - it should be at the natural waist level, and his jeans look like they're resting only slightly lower - and the guy in the back's head is a little lopsided on the top, though it looks like that's just because it's a preliminary sketch. I would also bring the line of the guy in front's neck out a little at the right; as it is, the curve makes it look like the neck goes in a little too far. Connecting that line to the bottom of the ear should resolve that.
Very nicely drawn, though. I'm interested to see how this is going to come together.