I try to understand the co-channel interference occurred in MIMO systems with rich multi-path channels, but I couldn't find clear explanation about how it happens and what's its effects to the received signal. I tried asking HERE about it, but the idea of the question was not clear.
Co-channel interference occurs when multiple users are sharing the same resource (e.g., time-slot and frequency) and is also known as multi-user interference.
In your original question, you were describing the MIMO channel - although not clear if that was for a single-user (SU) or multi-user (MU) system. If you meant a SU system, that wouldn't be considered co-channel interference. If both antennas are transmitting the same signal, you would be receiving multiple copies of the transmitted signal (i.e., transmit diversity) and that could reduce the effects of fading. If you are using two antennas to support multi-stream communication, I think you can assume there would be inter-stream interference.
Instead, if you considered a MU system (i.e., two independent receivers) that would indeed be co-channel (or multiuser, or inter-user) interference.
Note that this is intrinsic from the MIMO channel and it is modelled the same way as the intended channel itself. The usual way to treat this is to use precoders and combiners. In the SU case, for instance, the optimal precoder/combiner is obtained from the channel's SVD (so that you can orthogonalize the effective channel and suppress all interference). For MU systems, you can use zero-forcing to suppress all interferences.