A call to Custom Cartridge would get you an answer.
I wonder if they've ever worked loads for a double rifle before.

Pick up that phone Hoser - let us know what they say!

To get a handload to run right in any individual double rifle -- that individual double rifle will need to participate in the development.
Depending on the rifle/ammo/shooter, it might even be necessary for the intended shooter to be present for the load development.

Cheers
Tinker