With all the coupons Steam has at the moment, do they stack?
And I don't mean 2x 50% = free, I mean mathematically correct as in 100% - (100% - 50%)² = 100% - 25% = 75% total discount.
No, it's one coupon per item, and most don't apply until after the sale is over. The ones that do apply now are "golden" and say otherwise.
However, if an item goes on sale and you have a coupon that is valid during those dates, you can stack these two discounts, assuming it's not explicitly forbidden by the terms of the sale.
According to this help article:
Although it doesn't give a limit per transaction, so I can't vouch that you can buy two items and apply a coupon to each.