Two ashes on top of each other is actually possible ... I've done it :O
oooh I see!! I didn't undertstand your problem at first sorry. Yeaaah you can't place ashes on top of ashes as you said it, but there's a way to achieve it! hint below: (and thank you for your kind comments!)
hint: as you can't place ashes on ashes, perhaps you will need to place the two cards on each other before they become ashes