Generally as a rule it is a Jap Stag sireing a Red Hind that causes the Hybrid. Japs never have twins, or if it does happen it is extreamly rare. Selling hybrids I would think may be illegal ! just guessing here. But when the rut is on even keeping Fallow and Reds close togther could be a recipe for disaster.
I have dealt with several parks over the years with very large Reds, and when they are in rut, they can be bloody dangerous animals to be around, most places saw the antlers of prior to the rut, and sell it to the Chinese.