I did the "stack similar" a few times to shorten my artifacts list. But, then when I used an artifact for a +5% weapons increase, it seemed to use both of the artifacts. I only wanted to use one. Does that mean that all of my artifacts that are stacked will be used together instead of individually?

It should only be using them individually, this is probably a glitch

You cannot use two of them. They won't allow it anyway even if you double-clicked it.

It did use both. Not sure why. Both were gone from my artifacts, and when I moused over my attack thing, the effect was listed twice (one little box below the other). I don't know if it stacked the effects (10% or 5%).

But, since then, I've used single uses from artifacts (energy cube and the prejor scanner). So, must have been a weird one-time glitch.