Well, if you look at the beads, you should be able to see space between them. But, you're not supposed to be looking at the beads, so don't worry about that.
"Stacking" the beads is one of the biggest myths in shotgunning. If you want to look to see if everything is lined up correctly, and you've set your gun up so that the beads have 1/8" between them, then if when you look they have 1/8" between them, guess what? You're lined up just like when you set the gun up.
While you're at it, take a black Sharpie and blacken both beads.
And then go shoot at some straight-aways from post 3 to see how you're breaking the target with your new set up. Make changes to your comb as appropriate. (And don't worry about the beads "stacking.")
Have fun.