Front trigger first. Bore size has nothing to do with it. If your rifle doubles when you fire the front first, you have a problem with your rifle.

If you'll notice, your front trigger has a slightly lower trigger pull than the rear. This is so that you can more accurately place the first shot, with the rear trigger intended as a back up under duress. Also note, that on all double rifles that have a set trigger, it is always the front trigger that has the set feature.

Read the old African Ivory Hunters writings. They always talk about wearing out the right barrel. When purchasing a used double rifle, are you not instructed to pay particular attention to the wear of the right barrel?

Shoot it anyway you want but there is a proper way or at least an intended way. And that is front trigger first.