Double rifles are designed to be shot right/left. That is the way they are regulated. The best way to see what your rifle is regulated for is to look on the barrel flats. Depending on when the rifle was made it will give you the Cordite load and bullet weight the rifle was regulated to shoot.

I would suggest using the formula set for by Ross Seyfried in reloading Nitro Express cartridges. I have an emailable copy of this article and will be happy to send it to you if you would like. Please email me at rkmojo@aol.com

Congrats on your new doubles.
Take it over to Kona and hunt with Pat Fisher for some pigs on Parker Ranch!