It looks to me like the artist isn't trying to portray tribbing; Yukiho seems to be grinding against Alisa's pubic bone, and Alisa is just into that, rather than specifically getting off. (My current working theory is that the whole Japanese cultural construct of sexuality, including the whole seme/uke, tachi/neko thing, means that Japanese creators are a lot more comfortable with only one partner getting actively pleasured than Western ones do.)