generally if you introduce conflicting port forwards in your router, bad things happen.

Depending on the router's implementation, it might forward to both (not entirely desirable), or it might only forward to the second one specified, or it might detect a ruleset conflict and decide not to guess, and instead simply drop the packets it would be forwarding.

If you want to forward port ranges to two separate computers, you will probably need to pick two non-overlapping port ranges. Like my setup has 3 computers that might use bitttorrent, so we've got 6881-6989 forwarded to one computer (not that we'll ever use 100+ torrent ports at the same time). We've got 6990-7100 forwarded to the second computer, and 7101-7200 forwarded to the third. By specifying the port range ot listen on in your client, you have complete freedom which ports your bittorrent client listens on -- though it's highly recommended that you avoid the first 1080 or so ports, and you be aware of any services you may be running.

To specify the port range in azureus, just go through the config wizard, it'll ask you for a starting port. Generally if you give it 20-30 ports or more after the starting port to choose from, it'll do fine with that. The 6881-6999 thing is just the "default" range, it doesn't mean a thing in the grand scheme of things and it doesn't matter too much what you set it to.