Basically a double shot is a plastic cap with a second plastic injected into it to create an image or letter(s). The benefit is that the image will never wear out. You can even sand the cap down and it will stay.
Unfortunately only abs can be double shot, due to pbt having shrinkage issues when molding.
As for what caps can be doubleshot, pretty much any. I myself have an alps double shot cap. Just depends on if someone gets a different style made, which is hard without a group buy.