Signal combining refers to the addition of multiple signals (from one or more operators) to a single output feed leading to a single or multiple antennas. The most important specification is the isolation of the inputs from each other, or non-interference.   Combining is typically accomplished through the use of hybrid circuits which enable system designers to distribute signals from multiple carriers with only minimal insertion loss. CA-84N is a typical hybrid ombiner.
Signal mixing refers to the interference of signals as one would desire in a modulation system, usually using a non-linear device. Obviously this is most undesirable in wireless signal distribution systems.