I am planning a piece of jewellery that incorporates both gold and silver (both above the hallmarking weights) and I am wondering how to go about hallmarking the work.

I have a makers mark with Birmingham Assay but haven't made anything "mixed media" before.

Do I need to get it hallmarked with the gold or the silver (which will weigh more) or both?

There will be plenty of room for good sized marks on the reverse of the piece so I'm not concerned with the look.