disparate system communication

Hi,

 we  have a two disparate system let say ( x and Y), I have a set of list in X application in a List box which internally has key value pair such as ( _A123, Corporate), ( _B123,Office)

The Y application has List also with key value pair such as (1, Corporate), (2, Office)...
Let me know how i can map to x application  Key ( _A123)  to Y application Key (1) so that common corresponding Value
"Corporate"   or   can be the output...
 we use Xml as a Mediator between two disparate systems...
How we can  map this is my question....can this be achieve through xpath....
Let us know or any other way....

Thanks