Nick,
Welcome to the forum !!!!! There is a wealth of Double Rifle knowledge here. I agree with John, “The Chapuis 9.3x74R is probably the best bet and easiest to start off with”. I own and have owned several doubles by Chapuis (including a 9.3x74R) they all shoot well and have given me no problem. They are a great buy on the used market and there are plenty of “9.3 components “ out there to load it with.
Good Luck !!!!!!!
Robert