I've been wondering about that too and did a little research and came up with this:
"Alloys such as brass and bronze can be plated too, by arranging for the electrolyte to contain salts of all the metals that need to be present in the alloy."
So I guess the alloy forms on the surface in the same proportion is exists in solution.
Here's another technique that uses heat to transfer brass from a brush to steel:
https://www.youtube.com/watch?v=mFK9H4AG_Ww