Sounds like a sear problem. Whether you fired the front or the rear trigger first, or whether you had a soft or solid in the left barrel is a non issue and certainly not the cause of your failure. This is exactly what a double rifle is made for. You should be able to fire your right or left barrel first on demand.
Reason why I asked, I have seen front triggers on Merkels bend and gets bent so much out of shape that they eventually touch off the rear trigger, through no fault of the shooter, causing a double discharge.
Had this happen to a friend's 470 and my triggers were starting to go on my 500. Luckily noticed mine while rifle was having a new bead put on. This was due to a bad batch of triggers fitted to Merkels, as I have read of quite a few of these instances. Subsequently, replaced my triggers on the Merkel.
Saying all of this, if your mishap occurred from firing the rear trigger first, then it won't be the trigger issue, but I would have a careful look at them. By the way, got zero service from Merkel on the trigger issue. Could not even get an answer from there where to purchase new triggers.