I believe it would be best to join the two above the bay. When mixing in the ER the line from ER to the bay becomes depressurized of air and the liquid "settles" when selection is not in use. It will take some extra time for the line to re-pressurize and have the desired product in the bay in a decent time.
However you can have the valves pretty much where you want. Above the bay or in the ER. The difference in the valve placement is barely noticeable. Above the bay is ideal for the customer but when it comes to servicing and even troubleshooting problems -I prefer the valves in the ER for easier access. fwiw
I've seen a severely overcomplicated foamer design that had a 4 x 8' board inside with the foamers and it ran in 1/2" ID hose to the bays. It worked pretty well except for the farthest two bays which could take half a minute to get foam.
I'd always tee it as close to the boom as possible, no reason to make the customer wait for foam when he's paying, plus two lines of tubing are cheaper per foot than 1/2" hose.