yeah that's kinda what it does. It sets up two tsnet servers connected to two different tailnets and exposes them through a shared TUN device. It does the magic of allocating safe IPs to each host so that they don't overlap and does some leasing so that IPs aren't accidentally reassigned. It also does the MagicDNS thing so that DNS returns the "effective IP".

I wanted it to work on macOS so couldn't rely on network namespaces.

@bradfitz: Steal this idea ^^^ ;-)