In the UK, the hallmark applies ONLY to the “lowest” metal, so an
item that is silver and gold can only be Hallmarked as silver. In no
circumstances can a gold mark be put on silver.
Years ago, before 1975 (when Pt was declared a Precious Metal and
had to be Hallmarked) it was not uncommon to see a ring stamped as
gold and platinum, but nowadays it would only be stamped gold.