Your computer's creating a bunch of connections for each torrent - every person you connect to, to download creates a connection. When it's just your computer, it's just fine - no one else needs connections, so the router gives them all to you.
Then you connect the other computer to the network. And someone else needs the bandwidth/connections. So the router splits it up - instead of you getting it all, they get some of it. But not enough because you still try and use the same amount as before.
Lesson? Limit the # of connections and your upload/download speed. It's not the program's fault. You just didnt set it up properly - a common mistake