i found a resource that helps explain new brasses: http://www.afsinc.org/content.cfm?ItemNumber=6917
Basically, red brasses are replaced with new alloys containing bismuth and small amounts of selenium to replace lead but still maintain machinability. the tin, zinc, and copper amounts seem similar to old red brasses. the new yellow brass is still very high in zinc, and is separate from new red brasses.
it says casting new red brass is the same requiring no modifications to the process or part design, but bismuth improves hot tearing characteristics, and machining requires cutting oil more than leaded brass. hope this helps, but i still dont know if combining them does anything.