Joining 2 unfinished download

[hanugro]

Here is the scenario folks. My friend and I are exchanging torrent download and accidently we both downloading the same torrent recently. Now his torrent is 80% and mine is 25% so I am thinking about a way to join the two download together, and hopefully come up with more than 80% (close to 100% ;) ), so we can save bandwidth and time (let one of us to continue downloading the rest). Is there any way we can do this? Thanks in advance folks!