A uniquely decodable code is derived for multiple-access in a two-user interference channel, which is defined as a channel with complex input, output and arbitrary phase difference between the two users. The uniquely decodability is achieved by designing a trellis-coded modulation (TCM) pair, that permits the Viterbi decoder to decompose the composite received signal into the two constituent signals uniquely in any state of the noiseless interference channel. Theoretical proof is given, the result is confirmed by computer simulations.