you say you're behind the same router?

that could be a problem. If you're forwarding ports, you're only forwarding them to one of you or the other. that whole "port forwarding" thing maps ports to one or the other of you.

I work around this with my roommate by using a different port range. My roommate has 6881-6889 and I have 6890-6899. If he needs more ports, he extends down. If I need more, I extend up (you need one port per torrent you're downloading on). Your client should support changes of port ranges, so ..... try that.

You could do that, or you could just switch back and forth as the need arises, or have one of you be the designated torrenter. But with only one port range, forwarded to only one of you, you're not both going to have good things going on at the same time.