this is a really interesting idea. the using of two different spikes to create a gold compounded makes sense, but why not just use a ferring that can store identity and then give them a spike of someone else's identity who could compound gold instead, to where they would have, say, Miles Hundred-lives's identity instead of their own, and not have to worry about multiple spikes interfering and about Harmony controlling them.
bit of spoiler: other than using trellium to avoid Harmony's controll