Definitely recommended to have a sprung seat for the stoker (they can't see bumps coming)
Also 20-30km is the recommended distance for breaking in rides. You do want to get the positions sorted out before that though.
As for the gap in the middle - Brooks are hard and conform to you. Other saddles are hard and don't conform, so they add padding which applies pressure where you don't want it, so they have to add gaps in the middle.
Anyway, that's what I have found out from my research - got my saddle today (and no way am I going to hit it with a hammer. But then I don't have to ride on it straight away)