The old MoD brass cantle sporrans were attached to the bags with snaps. If you are using one of those, it should be easy enough to just make two bags to attach to the same cantle.

Brass is generally associated with daywear but really there is no reason why you cannot wear brass for evening wear -- just keep it polished! I'd base my decision upon what you think you have the most use for in your wardrobe.

Here are some brass cantled sporrans of mine, on leather and fur bags, if having a visual aide helps.

MoD cantle on buff deerhide bag (this has become one of my favorites and gets lots of wear):


Non-MoD cantle on dark brown deerhide bag:


MoD cantle on sealskin bag (I haven't had the occasion to wear this one since I obtained it, but hope to break it in at a Burns Supper next weekend):