While this is not quite answering the question asked about 2 cartridges specificaly I would say that there could be a problem with short cartridges in longer chambers.
One of my clients brought a backbored 3 chambered gun to the ground to try out. Using 2 fibre wad cartridges he put several large dents in my pattern plate (re-cycled fuel tanks). One of the dents had the remains of the shot load still stuck to it it formed a saucer shaped disc about 1 across and about 3 - 4 pellets deep all welded together. This was at 25 yards.
This is not anecdotal this is fact so ignore it at your peril.
I guess the problem lay with a combination of things. Long chambers relative to the cartridge, backbored barrels and fibre wads.
I imagine the 2 cartridge does not have a long wad so be cautious!!
John
