Why Just The One Letterbox?

[Toumal]

Frontier could easily expand the amount of station variety and include stations with two letterboxes, so that one would be exclusively for ingress, the other exclusively for egress.

 

[GG7]

An easy reason to see. The opening is the weakest part of the station, why would you make 2 weak places in the vacuum of space. OH my bad, for your convenience.

In addition to that...Only the Coriolis design would even be capable of having a rear airlock/exit/slot or whatever you want to call it. None of the Orbis type stations can have a rear airlock simply by design. Just take a look at the barrel the next time you dock at one. Where would you put another slot?

 

[Tarman]

A doubletoaster would likely be better as a new station variant than a replacement for existing stations.

 

[Flavourless]

It must be called :letterbox for a reason
But I never saw a postman trying to take the mail out through the slot where it goes in.