In this paper, we investigate the benefit of Joint Network Channel Coding (JNCC) for a new class of Multiple Access Relay Channel (MARC), defined as follows: (1) Independent sources communicate with a single destination in the presence of a relay; (2) The relay is half-duplex and applies a Selective Decode and Forward (SDF) strategy, i.e, it forwards only a deterministic function of the error-free decoded messages; (3) The sources are allowed to transmit simultaneously during the listening phase of the relay, but are constrained to remain silent during its transmission phase. We refer this model to as Semi-Orthogonal MARC (SOMARC), which leverages better the broadcast nature of the radio channel than its more conservative orthogonal counterpart, referred to as Orthogonal MARC (OMARC). All links are subject to slow fading and additive white Gaussian noise. This paper aims at measuring the advantage brought by the SOMARC over OMARC from both an information-theoretic and a practical code design perspective.