I would be very, very surprised if you can tap on at Weston UPX and tap off at GO Exhibition. I know it works that way for GO trains and it figures out the interlining/transfer through Union, but the UPX Presto machines are different and the prices aren't
exactly GO fares. If they say you only pay $5.02, while I'm surprised I'll believe it, but I think they might be making a mistake regarding not having to tap off UPX and then on GO at Union. Might be worth trying out.
Edit: Checked @GoTransit's tweets and
found another from your conversation where they say "No default - must tap on GO
@PRESTOcard machine at Weston even if you are taking UP train; must tap off at GO machine at Ex. ^ML" Huh...that's very interesting.