Is double teaming secretly illegal or at least considered cheating?
Like getting one person at the other end of the table raising and raising while you are the one that makes the kill.

I don't know, neither do I have a buddy for this this, but it looks like some interesting act :)

Do you mean like what took place in the movie, Rounders? In this movie they were passing tells to each other. I would call this cheating.

Thats cheating unless the other players also know about this and are ok with it.
If you do something like this it is cheating and immoral.

Its pretty shady to run tables like that, an unfair advantage over the other players.

I definitely think it is cheating.

I know people do it and players really aren’t happy if they find out someone at a table is doing it, I know I really wouldn’t be happy!

It's illegal. As per house policies partnership or any kind of co-operation between the players is prohibited.

The term for it is 'collusion', and apparently the Poker rooms have software to detect betting patterns and discover people doing this.