There are a few ways to do that. One is to make your own with the template and the other is to set several different tattoos and "cut out" the other section of an exported MainTex to one with both. I cant show every step with screenshots but I have a few quick examples I did with paint.net, but the prosses is the same with any image editor I'd assume. As far as I know there is no mod for it but I could be wrong.
The first one is to get the overlay template from the option at the top of the general tab. That will export a wire mech png that you "draw" on top of. Then you just de-select the wire mesh layer and save the others as a png that you then add to the overlay as you would any other.
As for the MainTex, you can use both paint slots however you like and then in material editor you can export the MainTex, the clear the paint slots and use whatever you like again and export that one. Then in your image editor you cut out the section of the top layer where the other paints are to make a MainTex with both. Then import the new one. The MainTex will have an asterisk to show it has changed then save the character as a new card. It's a lot more involved.