If they are running on the same system, he can exchange data using DDE.

For example, if one instance is named "mIRC1" (/ddeserver on mIRC1) and the other instance is named "mIRC2" (/ddeserver on mIRC2) the script on "mIRC1" can send the following request to "mIRC2":
  • //echo -a $dde(mIRC2,evaluate,% $+ announce)
and "%announce" will then be evaluated on "mIRC2" and the value will be returned to "mIRC1".

For more info: /help DDE