Multi balls do seem to stay together, I don't know why, perhaps one is sucked along in the slipstream of the leading one.
The first time I noted this phenomenon was in the early 1960's when I used a single shot .410 shotgun for rats and rooks. As an "experiment" I removed the lead shot from a cartridge and put in two hard steel .375" ball bearings. I took the precaution of rolling the balls through the barrel to ensure that they wouldn't jam in the choke and than weighed them to be sure that they were no heavier than the shot.
I fired this cartridge at an upside down scrap motor car at a, paced out, distance of 40 yards and then walked forward to see if I had hit the car. The balls had struck the chassis below the door and passed through five layers of pressed steel before coming to rest in the sixth steel panel. The big surprise came when I used a crowbar to dig the balls out. They were stuck together in the steel!
The rear ball seemed to be undamaged, a micrometer showed less then .001" variation wherever I measured it. However, the front ball had a deep spherical depression in the rear where the other ball had hit it and was expanded to a larger diameter. I still don't know why one ball was still spherical whilst the other was "squashed". I wish I still had those two balls.
Anyway I decided that firing two bore-diameter balls, of whatever material, was a waste of time, use a heavier bullet or multiple smaller balls.