How do I join two tables on two different instances of DB2?
The "federation" feature in DB2 will allow you to do what you need.

It's available on the Linux/Unix/Windows version of DB2 - not sure about the mainframe - and there are no licensing issues, it's part of the base product set.

To implement it you only need to create a few extra db objects (wrapper, server mappings, nickname) and you're more or less done.  Follow the documentation and you should be fine.

Cheers.