I hate portmanteaus.
Seriously.
John Locke is this guy
Or this guy
And I don't even want to get started on Janto. It looks like something a kid would say.
But mostly, people have separate identities outside of their relationships. :D :D :D They are not halves of a whole! (And don't even tell me that it's hard to type those extra 5 or so letters. Seriously.)
(Another thought - the term "slash" comes from the little '/' between two characters' names. When you use a portmanteau, you're writing, IDK, meld.)