Nice idea if you actually have the rest of the redundant network, uplink and all that jazz (otherwise you’re wasting time and money).
the reason this won’t ever be a product is because if you’re serious about your redundancy you’re installing extra NICs inside the servers, which are ideally not second-hand. the only people who would be the target market of such a product is just you.
also: do these servers not have pcie slots inside? is there truly no way of adding nics inside?