System Information Block-1 (SIB1) broadcasts only one tracking area code in cell. It means one cell can't be part of two different tracking area. SIB1 can also carry max to 6 PLMNs. All these PLMNs can belong to same MME or different MMEs in case of network sharing.
As per my understanding of cell level and SIB1 details, if a cell is being shared between multiple operator then both the network will have to consider tracking area a common tracking area. A single cell can't belong to different different operator's tracking area.
I may not be correct, lets others to respond.