I've had my suspicions before but after viewing Mercy_Rabbit's Pixiv page (http://www.pixiv.net/member.php?id=332098 ), I'm certain that he and Marcy_Rabbit are the same. I'm not sure which should be the alias and which should be actual artist name. Also, I believe that the link in Marcy_Rabbit's artist entry leads to a publisher and not his own site.
Updated by KeliraTelian