Judson:
there have been a few threads on this topic before, some quite detailed and one in particular a little lively! You might profitably spend a rainy Sunday flicking through all the old threads on this topic and reading any of interest to you. Here's a head-start:

Two-pager

Another...

and Another

Lively three-pager!

and one on Backthrust.

Quite a bit of the previously-published stuff is speculative, and has been heavily influenced by Ellis-Brown's text. Even though a number of members have built their own, and hunted with them, any additional hands-on experience and first-hand advice is highly valued. I would be interested in your views on 'moment of inertia' and how to retain the quick-handling characteristics so important in designing a working SxS double. Many US 'smiths appear to be reluctant to slim down the barrels towards the muzzles, which is crucial to the 'liveliness' of the finished rifle.