I know you wanted to talk to those who have done multi-layer signs. I did one and was going to place rivets with spacers but ended up riveting without spaces. Looked great. My original idea was to use aluminum spacers with 1/4 inch ID and rivet front and back pinching the two layers together.
@DonP does some wonderful projects with a stud gun. Assembled after painting.
@Wsidr1 does some exceptional pieces by epoxy fixing bolts to the backside of the front piece and uses nuts to space as desired. Assembled after painting.
Others talk of some welding scarring the metal on the show side.
Hopefully, they will weigh in and clarify. Looking forward to seeing your results.