That sounds like a difficult one; I doubt livecycle will send both types in one email.
I'm not too sure, but perhaps (as a workaround) a normal button could have two events scripted to send in different formats.
e.g. on a standard button the mousedown event could be coded to submit as xml and the mouseup as .pdf?
If that worked (and i have not seen it done) that would generate generate two emails though - and of course the submit as .pdf requires the form to be reader extended or rendered with forms server, or the user to have acrobat.
good luck
