Front trigger first, as you would typically with a double shotgun.

I don't have any problem with just the "tip" of my trigger finger on the front trigger, but use the first joint on the rear since the closer trigger feels more natural with more finger on it.

Try the snap cap in the left barrel to make sure you aren't loosing control of your hand and fingers under recoil. This is the cause of (most) doubling, and it is only correctable error.

I'ma a lefy shooting right actioned rifles and shotguns, and so my trigger finger extends across the rear trigger. If anyone was going to have trouble controlling their hand and so trigger finger sufficiently and causing a double, it would be me becuase of the right/left trigger location. But tens and tens of thousands of rounds through double trigger guns, mostly shotguns, have yet to produce a double.

JPK