Very good question.
I would try to save the data (if it is just a small text or so) using qa_opt("data_storage_name") by plugin n°1.
And then read the data from plugin n°2.
Note: The executions of the plugins is determined by the folder names (in q2a 1.6.3-), as far as I know. So plugin 1 should appear alphabetically before plugin 2.