He would be back-up because Kane is currently front page news across the country, and is the new face of the team.
Also, there's the matter of 31 goals in all competitions.
Kane didn't spit his dummy out because Soldado came in a big money deal and was supposed to be "the guy". However, Kane is now "the guy" in a fairly visible way. That changes things, and having the back-up coming in on higher wages would either mean the club doesn't think he's going to be the main guy going forward, or he's underpaid and should be demanding more.
I don't think Harry is a problem child likely to demand a massive salary. But I do think that we would need to pay substantial wages to Benteke to convince him to come to Spurs as a back-up to Kane, because there is no way he wouldn't be the back-up.
I don't think that's a can of worms worth opening, especially when I don't think he's worth spending 20+ million on when our needs are much greater at other locations on the pitch.