A lot of people (notably Robert Borski) suggest that Tartaros is in possession of Pig, while Michael Andre-Driussi, in discussing the possibility of Sev. as one of Pas's sprats, suggests Tartaros.
No to both, for the same reason. Tartaros is not only blind, was not only born blind, but is utterly and incurably and inalterably blind; even when riding/in possession of Auk, he cannot see through Auk's eyes but must ask Auk to describe things.
So he isn't Sev., unless you want to consider "loose traditional reincarnation," of the sort by which any two persons not alive at the same time might be the same person -- which is to say, by which it would be meaningless to talk about "same."
And he isn't in control of Pig, and Pig certainly isn't him; Pig can see when given an eye.
*This is WHORL, for discussion of Gene Wolfe's Book of the Long Sun.