Transferring large numbers of items using the current trading booths is a nightmare. I suggest enlarging the trading booths to allow for an extra dispenser so that items can be transferred quicky between 2 shulkers. Also, if 2 or more people are in the room, it's really difficult throwing items to each other, it's impossible to say who's gonna pick up what thrown item.

I dont think that they are supposed to be used to trade items or to have more than one player in them at a time as that's what the trading booth is for