I think that it's safe to load only 2 balls in some smaller rifle caliber muzzle loaders like the .45 as long as the powder charge is reasonably less than the maximum and the two balls are properly seated by touching together right on top of the powder charge.
I know that I've done it and many others have done it from rifled barrels, including Smokin_Gun with his .50 rifle and 60 grains of powder.
But it's very important to know your gun and its limitations.
Of course anyone who does so does it at their own risk. But the powder charge must be reasonably moderate. I wouldn't do it in a large caliber gun or in a barrel without thick walls.
No one wants to get hurt and overdo it with too much powder or too many balls.
And no one wants to ruin or bulge their barrel.
There's ammo manufacturers that make 2 ball commercial loads too and they're not just made for smoothbores.