CSY Query <serial commmunication>

Hi,
I have a doubt/Query.
It is in serial communication.
Condition:
There are two clients <"A1" and "A2"> opening the port in sharable mode.
Both clients have their read request
Both uses same CSY.
The data comes from H/W.
Data is received by CSY from H/W
Now, Who decides it, which data to go for "A1" and "A2".

Plz tell me the way or any API if known!!