I like that they have substantially different thrusting speeds. It makes them seem more distinct from each other, even for how little we can see of them, avoids the entire thing looking unnaturally synced, makes it seem like the two are different distances from orgasm (simultaneous stuff is a bit overdone, in my view) and overall makes the animation much more interesting as a whole.

Content-wise, it's not really to my tastes, but I always appreciate seeing some well-done animation. Sorry for the analytical tangent.