This is an important article to read if you've every wondered about how to get add-ins you've written to talk to other add-ins loaded in the same Office process.

http://blogs.msdn.com/vsto/archive/2008/05/07/calling-into-a-vsto-add-in-from-a-com-smart-tag.aspx

Basically, it involves the callee Add-in overriding RequestComAddinAutomationService to provide an object that caller add-ins can talk to.  Then the caller add-in uses the ComAddins collection off of Office Application object to find the callee add-in it wants to talk to, calls .Object off of ComAddin, and the caller add-in then gets the object provided by the calee Add-in's implementation of RequestComAddinAutomationService.