I have always carried my double rifles with a soft in the right barrel, and a solid in the left while hunting buffalo. The one time I was charged un-provoked, from close range I was carrying a 500/450 NE double rifle loaded that way. I still fired the right barrel first, aimed at the chest, followed close behind by the solid in the left barrel aimed at the brain!

That bull took a 480 gr Woodliegh soft in the heart, similtaniously the heart also took a 300 gr soft point from the PH's 375 H&H bolt rifle. Both shots hitting the heart, seemingly without effect being visible. The next shot was the 480 hitting the bull in the face scrambleing the brain, with another 300 gr solid hiting the face off center, because the 480 gr solid hit it first turning the head slightly. The second 375 bullet went into the cheek of the buff, exiting at the back of the bull's right jaw, and re-entering just inside of the right shoulder, and skidding along the ribcage stopping between the scapula, and ribs.
All four bullets hit the buff, and the first two were lethal, but not stoppers, and if both the second shots had missed the brain, one or both of us would have been in big trouble. If the last two shots had been reversed and the brain had been hit by the 375 the result would have been the same. The reason the second 375 bullet hit off center is because the 500/450 fired the second faster than the 375 was because the 375H&H was in a BERNO bolt rifle, and was fired a split second later than the double rifle chambered for the 500/450NE.

I have fired my 470NE Merkel double rifle both ways many times and it has never caused a double discharge, nor has the Merkel 9.3X74R double doubled but once, and that was caused by a sweaty trigger finger, slipping off the front trigger, and hitting the back trigger, and I felt it when it happened! I have also owned a number of S/S Merkel shotguns as well, and have never experienced a doubleing with them either, no matter which barrel was fired first! I personally do not think this is a problem with Merkel doubles, but I do think it is a problem with shooters!