You can’t mask more than one thing to an item. Don’t know why, but you can’t. But there is a way you can make it look like more than one thing is masked.
Say you want two tattoos on a head. You mask one tattoo to the head. Then, you make a second head, totally identical in size and place, and mask the other tattoo to that. Then, you make the second head totally transparent, and you can bring it down to the bottom layer, if you want.
Also, if I understand it right, you can’t do the second one, either. Just make make multiple of the tattoo, exactly the same in shape, place, and color, and mask each one to a different body part, but in the exact same place, so it looks continuous.
I hope this helped a little.