I'm no framebuilder, but even if you have the proper lugs, I'd think there is some extra work to attach the second seat of stays. The dropout region gets a bit busy, and there is a third attachment point on the seattube. So it's likely to cost more regardless.
Peter Weigle makes (very nice) mixtes. There was an article on Bicycle Quaterly about
this one he made. MAP bicycles makes some too and there are surely others. Still they don't have the twin stays from the headtube you wish. In that BQ issue the different geos of mixte bikes is discussed too.