I'm with you -- IMO *BOTH* medications should have warnings on them about incompatibility with the other product.
Interestingly, I asked this question of the tech support people at Seachem (Cupramine) and Novalek (Amquel Plus). I've read all of the product literature for both products. The Amquel people pointed me to their product literature and two articles relating to their copper products. (I actually felt like I was getting more of a sales pitch from Novalek, trying to sell me on their copper based remedies, rather than a technical answer to my question).
Neither manufacturer mentions that their product is incompatible (in a deadly fashion) with the other manufacturer's product.
Neither manufacturer mentions that their product is incompatible (in a deadly fashion) with their own similar product.
Neither manufacturer even mentions that their product is incompatible (in a deadly fashion) with an entire class of drugs, even though their product literature contains a section entitled "contraindications."
Causing fish death through poisoning is clearly a contraindication to the product's use. This represents a serious labeling error that needs to be fixed.
If I didn't have a degree in chemistry and a nerd's interest in understanding the mechanism of action of the drug, I'd have several tanks full of floating fish right now.