I have ribcraft 4.8 with one jockey seat.
Intend to make up my own T shaped bench/ jockey seat with a removable back rest on the jockey seat part. This way the rider has something between their knees, and a back rest to brace themselves against. And I get plenty of dry storage.
I don't think there is enough room in the stern for a 2nd jockey seat and a separate bench seat/ storage box as well. Not in a 4.8.
A bench seat alone would be hell for a passenger going fast in rough water - they need something between their knees OR plenty of room to move their feet and brace themsleves properly when standing up. And with a bench seat behind them, there just is not enough room.
A 2nd jockey seat is fine for the ride, but doesn't give enough space to store things.
So far as balance is concerned, I have an under floor tank. With 80 litres of extra fuel in plastic cans, and 12stone of passenger, I can store things in the bows and the ride is excellent. With half the fuel gone, I have to move some of the stuff in the bows further back, or be careful with the throttle/ trim to stop the bows digging into the back of the wave in front.
Personally, I would not want to sit on a front bench seat in a 4.8 RIB when under way. For storage it is easier to lash things down under a tarpaulin with a 'cargo net' of elastic cords over the top.