I know they can be used for subclass quest to exchange for low A weapon, not sure of mammon though. Try it, if it fails, you sell duals and buy whatever weapon you need for trade.
Key items and recipes are a pain in the neck to get a hold of on Lionna - AoBA blades are the only ones I've recently seen in sufficient quantities, and they are 580k each from that store. Buyshopping is pretty hopeless as well.
You don't need MA or Arti frames for dual SLS by the way - just the 555 SoP (~2.7mil), a bunch of crystals (~13.3mil), and a bit of AA (4.4mil AA with GSS at 6a/each = 5.3mil), arriving at a neat make value of just over 20 mil. :p