Can $n=pq$ be part of two different pairs of RSA keys?
If such keys exist, say $(e_1,n)$ and $(e_2,n)$, how are they related? What will be the security concerns for the two users?
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The users will be able to read each other's messages (even though they can have different private keys, say $d_1$ and $d_2$). This is because knowledge of $d_i$ is sufficient to factor $N$, thus allowing that party to compute the other party's private key. This was detailed by Boneh in his analysis of RSA attacks.
Two properties of RSA are important here:
This means if you know one private key for a given $n$, you know all of them. Thus different persons should not share a modulus.
Such a scheme can be useful if one person needs multiple public keys. This person can use multiple $e$s with a shared $n$. There are few restrictions on the choice of $e$s, for example $ e_3 = e_1 * e_2 $ is broken.
Such key pairs are only useful in very specific scenarios. For example some blind signature schemes use different values of $e$ with shared $n$ to sign different currency denominations.