I was just about to ask about this. Since the sale last until the 22nd I will hold off buying it for now, but unless there is a concrete answer before that I'll probably buy it, in case the coupon doesn't stack/isn't out on time. A 25% discount is much better than a 10% discount after all (a 35% discount is even better though).
I thought the code just upgraded the base game for free to the Obsidian version? I doubt the coupons will stack so I'll be getting the base game and hope that I qualify for the code. I have the iOS version and the full Rise bundle but I bought that a long time ago, no idea how much I spent.