I've actually seen only a few. ChickenFajita12 is the king of Co-op but he does it with an IRL friend, Confederacy65. Other than him? Topmonhit is doing a co-op of Blue, he's kinda popular I guess. It's very rare for Co-op LPs to happen, usually if it is it's only an episode or two (see MunchingOrange and StebenL, and that was before everybody knew who Orange was, JoltsBolt battle tower stuff and Jimmy31428 and Anniecarrots co-op)
And yes, the next two parts of Ash's quest are being uploaded after I post.