As you can read from the spec., there are no UE IDs in a MAC PDU, in contrast to the LCID subheaders that denote user data from multiple logical channels. It's implying that one MAC PDU can multiplex user data from multiple logical channels but it has no means to multiplex data from different UEs.
In addition if you pay attention to the channel coding process chain, there's only one UE ID (i.e. C-RNTI) to scramble the CRC for a DL DCI, which also implies that the MAC PDU is destined for one UE only.
However, technically speaking, there's one type of MAC PDU that encapsulates data destined for multiple UEs. It is the MAC PDU that carries RA Responses. The DCI for this type of MAC PDU is scrambled by a shared RA-RNTI. Each of the corresponding UEs, after decoding the whole transport block, only retrieves a section of the MAC PDU (called a RA Response) according to a Preamble ID subheader, discarding the rest of the PDU.
