I'm pretty positive that (A) is not possible.
(B) probably is possible since a button can more than one action. You could create a button which first stops playing the first MP3, then starts a second MP3.
Thanks, shame about A) in regards to B) I have managed to do this when 2 mp3's are on the same page, btu not when the other mp3 is on a different page...
Any other ideas?