Why are flying-boom tankers restricted to a single boom?
There are several reasons why it is hard to support multiple booms on a single tanker: 

1) Booms need an operator who is located in the fuselage. Video systems provide for remote control on newer tankers, but older ones still use direct view or looking straight down the boom at the refueling point in the receiving aircraft.

2) The boom is physically large. Its weight, large diameter and telescoping tube length would be difficult to install on a wing. 

3) A wing bounces around a lot more than the fuselage does. Since the boom is rigid, this complicates keeping a good connection. 

4) The high flow rate of a boom is only useful for larger aircraft like bombers, which are too big for simultaneous refueling. This somewhat reduces the advantage of having multiple refueling points. 

Probe and drogue has a lower flow rate through a lighter, narrower hose. It is passive from the tankers point of view, and it is flexible.