Zero knowledge of two factor Here I overconfident in myself state that I can show, that n has two factors.

This is not completely true, can possibly show $n$ is composite - prover generates RSA key with modulo $n$, and gives $e$ to verifier.

Verifier takes random number $x$, and checks if $x^{e*d}=x$, where $d*e=1$ (mod  $n-1$). If it is, then $n$ is prime, if not $n$ is composite and has factors.

Verifier sends $x^e$ to prover, and if prover sends back $x$ we can be sure that prover has factors of $n$. This doesnt, however garanties that there is just two factors.

How to prove-verify that there is only two factors in $n$?

UPDATE: This is not zeroproof, it is interactive here as was pointed in comments. Well, then. Question becomes more broader - can (duh) show that $n$ is not a prime. How to show it has two factors, zero knowledge, without interactivity?
Firstly, convince the verifier that $n$ is not a prime number. (Prime is in P)
Use Zero-Knowledge Proof to make sure that $n$ has no square factor (no $p^2|n$)

See this paper Practical zero-knowledge proofs: Giving hints and using deficiencies for details.
Showing that $n$ has exactly two factors: if not, the number of quadratic residues is roughly less than $1/8*n$. Verifier just randomly chooses a value y and ask prover to prove whether y is quadratic residue. 

If the probability is roughly $1/4$, then accept.
Verifier can't cheat since he can only compute the quadratic residue, and let the probability raise, but not decrease below $1/8$.