Lefty here and over 50 years of double triggers (both modern and muzzleloaders) has never been a problem. Stock cast-off for righty shooters is a big concern, but not the triggers.
Only place I run into issues with the triggers affects both lefty and righty shooters, and that's order of fire. Lotta lotta folks use the front trigger first, and if the guns are choked for that, the front trigger is usually the more open choke. But I was trained/conditioned to use the rear trigger first on heavy recoilers (large cal double rifles), so that a finger slip in recoil doesn't drop back and fire the second barrel as well. Can't break the habit, and it still gives me the willies to pull that front trigger first.
Thankfully not an issue on the muzzleloading double shotguns I prefer- Cylinder bore in both barrels, so it doesn't matter choke-wise which I pull first.
Fine tuning of style and form in my case, but that little 1/8" or so lateral spread between the front and rear triggers shouldn't be an issue for run-of-the-mill lefty shooting.