Or to both downvote the same things, or to comment twice on the same post in a way that makes it seem like it’s 2 people. Basically if both show up in the same space and you pretend that each is its own person, that’s probably sock puppeting.
Oh the other hand, it’s not sock puppeting if the two identities are active in different spaces. If you use one account to comment on PornHub and a different one to post on LinkedIn, nobody’s going to say that’s sock puppeting.