Debian was kind enough to configure their distribution's copy with a distinct hostname for the transit relay helper (the bit that forwards bulk encrypted traffic when both parties are behind NAT). "magic-wormhole-transit.debian.net" is currently a CNAME for "transit.magic-wormhole.io" (which is what the upstream source uses), so all this currently costs them is some DNS maintenance. Both sides exchange transit server hostnames, so they don't need to use the same one, but Debian does this so we could switch Debian-based clients off to a different server if/when my costs of running transit.magic-wormhole.io grow too large.

The "mailbox relay server" for all mutually-communicating clients must be the same. Both upstream and Debian (and most of the other distributions I've seen) use "relay.magic-wormhole.io". The mailbox server helps the clients exchange tiny key-exchange and setup messages, so its costs are trivial.