I've got a 150 mbit cable and a 40 mbit dsl connection, one one each NIC. I'd like to use both connections for torrents, so I assume I run two separate copies of vuze, one bound to each NIC, both with the -DMULTI_INSTANCE=true flag set? Then do I use a separate download directory for each or are they smart enough to use the same downloads directory and not step on each-other's toes somehow? I assumed they each download separate copies and simply exchange each completed piece with each other locally, but the same download directory would save a lot of space. Anyone set it up like this?
I have never heard of anyone doing this before. I have no real advice to give you at all.
Have you tried to do this with two different clients? I would start there . . . and if you can get that working then try to get it working with two of the same torrent client.
I assume you are on Windows but you did not specify that at all. I know you could not do this on a Mac. You can not run two instances of the same program on Mac (unless you do it in the Unix side of things).
If you have one instance of Vuze and configure two bind-ips under Tools->Options->Connection->Advanced Network Settings then Vuze will round-robin out-bound peer connections
For incoming you're stuck with one NIC though as tracker/DHT traffic will be routed by whatever interface your system decides is best.
Two separate copies of vuze pointing to the same files isn't going to work due to file locking - you'd be better off running them as separate downloads and, as you say, using local-peer discovery to cross-populate.
The machine I'm on now is Win8.1, I've got some Linux (CentOS/Arch mostly) and a couple BSD boxes (firewall, Plex server)
That's how I've done it in the past, use two comps or a VM so I can run two copies of uTorrent or qBitTorrent or whatever, each on only one connection. Then they discover each other locally and share the bits they each get with each-other at LAN speed. This'll be a lot easier since I can do it easily on one rig with just two copies of Vuze.