Handle the gun (at the range) as you would in the field.
Everyone I know and most I've seen who handle double rifles and combination guns / cape guns -- they grip the barrels forward of the forend wood.
If you're shooting from a bench, set up in a position that is as similar to your standing field position as possible. Grip the gun as you would, and if you must use a rest, use only a rest under the back of your support hand.
This way the rifle can recoil as it would naturally and will print your targets in as similar fashion as it would in the field as possible.
Have fun with it!
Cheers
Tinker
