I would imagine technically stacking them balls (or signed items) would be the same as stacking a, say, Dream Hat with a Warlock's Tabard.
In other words, they are different items.
I always thought them balls should be key items though, and I imagine it should be even possible with the limitations there are.
As much as I hate to get them since I am always full of more or less needed stuff, I always will, since if you don't play, you won't win.
Try at least I can.
That said, I'm not really too eager for more such things to be had but I wouldn't probably be against it: I would not mind.
