In this paper, maximal-ratio combining (MRC) and selection combining (SC) are proposed in spectrum sharing single-carrier networks with multiple primary user receivers (PU-Rxs). Taking into account the peak interference power at the PU-Rx's and the maximum transmit power at the secondary user (SU), the impact of multiple PU-Rx's on the secondary network is characterized when the secondary user receiver (SURx) is equipped with multiple antennas. In doing so, exact and asymptotic expressions are derived for the cumulative distribution function, taking into account two realistic scenarios: 1) non-identical frequency selective fading between the secondary user transmitter (SU-Tx) and the PUs, and 2) frequency selective fading between the SU-Tx and the SU-Rx. Based on these, exact and asymptotic expressions for the outage probability and average bit error rate are derived. Furthermore, an exact closedform expression for the ergodic capacity is derived. It is shown that the asymptotic diversity gain depends only on the number of receive antennas and the number of multipath channels. It is further shown that the number of PU-Rx's and fading severities between the SU-Tx and the PU-Rx's have no impact on the asymptotic diversity gain.