Seems like there are 2 of you wanting to do this. If I was going to do this from scratch, I might buy a Bond Arms barrel and design to fit. I am not sure if Bond can sell to France and Australia though. If so you would access to all there calibers.
The best approach is to draw something up, combined with a paper mockup of the overall shape and arrangement, then make parts.
You can find enough info to keep you busy for a couple of weeks at the various online home gunsmithing, garage gunsmithing, and similar sites where folks are building similar firearms. (Those are hints.)
A search at Google patents will also turn up more patent drawings than you can absorb in several months.
I have the prints for the Childs pistol and all the Jaco pistols, but none of those are two shooters.