Communication complexity of finding common element of two subsets
(Reduction from set disjointness)

Suppose Alice and Bob are given sets $S, T \subseteq [n]$ with the guarantee
that $|S\cap T| \leq 1$. Alice and Bob run the protocol for finding the common
element of $S$ and $T$ assuming that their sets have a non-trivial intersection
to decide a common element $X$. Now, they can communicate with each other to
verify that $X$ is in fact common to both $S$ and $T$ with $O(\log n)$ bits.

Therefore, any protocol for finding the common element of $S$ and $T$ must take $\Omega(n)$ bits of communication.