BB

Regardless of the rear sight, if you have the same sight picture for both shots and as above, you are shooting it correctly, it will still put both bullets where it is regulated.

So, don't worry on where they shoot on a page at this stage,
get them shooting together.

Do you have a copy of Graeme Wrights book, shooting the British Double Rifle ? If not, suggest you get a copy.

Might also be worth throwing the odd target up here
and loads and velocities. Plenty of people like NitroX,
Cpt Curl et al will be able to give good advice.