irrigation loops

What sort of flow control is required and sensible to place on a line between exterior spigot and an irrigation loop?

I want to create a loop between two outdoor spigots. From the loop, I plan to add vacuum breaker for each branch that draws from the loop. The loop would be underground. I know that one spigot in theory could push water from the exterior main loop into the house without flow control - which I want to prevent.

You have a bad "theory" and you CAN'T safely connect two hose valves to the same loop. Where, and what type, backflow preventers needed depends on the way you install the system so we would need a drawing of what you want to do, at least to the area control valves.