On the tx side of the uplink, I am assuming that you are able to create one resource grid for one UE for one subframe. In the context of the LTE toolbox, a resource grid is just a MATLAB matrix and let just call it G1 (for UE1). Most of the elements in this matrix G1 are zero except the elements used for the transmission of PUSCH and PUSCH demodulation reference signal. Then call the function lteSCFDMAModulate() to generate the complex baseband IQ samples from G1. Apply channel fading to this IQ samples.
Now do the same thing for UE2 (of course using different transport block) and you should have another resource grid for UE2. Let call this resource grid G2.
Now you can simply add these 2 IQ samples (going through its own channel fading) and this is your received signal.
Does this make sense? Am I on the right track?
Let try to do the tx and channel first. After that, we can do the rx side.