Try it, but use a smaller charge of FFg- say start at about 60 grains, with a PRB, and work up from there in 5 grain increments.
DON'T expect the two barrels to be regulated to shoot the two balls to the same POA, at any distance. These guns often will have ONE barrel that gives consistent accuracy with one load, but the other barrel needs a different load to get a small group. You choice is to custom load each barrel, or just load one with a PRB, and use the other for birdshot when you hunt.
The markings on the barrel indicate the Maximum load the factory is suggesting for that gun- NOT A RECOMMENDED LOAD. I personally think your gun can stand a load of 90 grains of FFg powder and a 1 1/4 oz. of shot, but a lot of pressure is put on the nipples, and their threads, with these kinds of loads. If anything " goes", it will be the nipples. I don't think you want them flying back towards your face.
Don't think a smaller powder charge is a "wimp " load, either. That heavy ball- usually more than an ounce of lead-- is a freight car you begin moving. At 50 yards, and even much further, it will completely penetrate a deer, expanding even at low velocity , from its .71 caliber, to over an Inch in diameter, before it exits. A soft RB is a much faster killer( assuming shot placement in the chest of a deer) than even a 12 gauge foster slug. You will hear the impact of the ball. :thumbsup: