I think they must be a subcategory. It looks like all Split OTPs would qualify, but what I'm thinking of are not necessarily Split OTPs. Like for example, in the Teninch arena, you do get a lot of "Rose + this other character David Tennant played". But then you also get "Bella from Bella and the Boys + Campbell Bain from Takin' Over the Asylum", which were never a pairing in the first place but seem to have been put together because the real OTP in the fanficcer's head is David + Billie.