Re: How can I say the seat which is next next him?
Welcome to the forums.
I do wish we could just adopt your suggest of "next next" -- it's a neat solution and no one should misunderstand if they thought about it. But unfortunately, I can't recommend it.
"Anna is sitting two seats to the left/right of Tom." -- That's probably how I would say it.
PS: Next time, please don't use abbreviations you would use in a text message, like "ur" instead of "your." This is a forum for the proper use of English.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.