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Is there any value to having 2 machines on the same network working on the same torrent, but, one of them using vpn (or proxy)?

My download throughput is far, far less than my total available bandwidth.  From what I understand there is a somewhat randomized selection process of which peer shares its pieces with which other peer.

The idea behind having 2 or more IP addresses externally, is to increase the likelihood of being selected.

If the 2 computers are sharing the same internet connection but 1 is using a VPN through it...

A VPN has bandwidth overheads of its own, due to its encryption, encapsulation, and smaller packet sizes. (Pay more, get less!)

Splitting everything 2 ways means neither half may be able to upload quickly enough to come to the attention of fast peers, thanks to BitTorrent's Tit-For-Tat.
At low upload speeds, peers will SNUB your 2 peers and then almost never upload to them even when they (the other peers) become seeds!

The 2 computers will at times be downloading duplicate pieces ...and probably not be able to share pieces with each other easily over the LAN due to the VPN. Quite likely they'll connect to each other over the internet through the VPN and have to pay a double cost for any pieces shared that way.

Even if duplicate piece downloading could be completely avoided...

On a poorly-seeded torrent there will likely be no gain -- peers there likely have no "free" upload to send your way no matter how many peers you add. What little you do get comes at the expense of everyone else.
On an over-seeded torrent there is no point -- you should be able to get way beyond 1 MB/sec download speeds anyway.