Victor redance
Yes I know it's back there when it's full of fertilizer and you point it up a hill. I'm holding 880 bushel. I'm using variable rate linear actuators on 6 rate transmissions and ag leader integra controllers on both carts and have fertility set up in zones. The rate on each product is adj independently of each other based on field prescription. All the fertilizer is kept separate in the carts and is blended as needed by zone requirements during planting. I've fed the drill around 650lbs total with seed and fetilizer combined in both dual shoot air streams at 4.5mph with a 60' drill with no problems. The smaller the drill the faster you could run. Speed limits me as to how much the drill itself can take. I have trimmed and routed the hoses on the drill to eliminate all sags and drops and make them as short as I possibly can. I happy with the job it does at 4.5mph and it is just easier on everything.
Also I can turn and wrap the drill in a circle and load both carts with two semis at once. Cuts my fill time in half.

I have two hoppers on semis and have a different fertilizer in each one.
Both of my carts were dual shoot to start with so it made plumbing a bit more simple