It would not be easy because the foil would be very thin.
The oxides on their surface layer is normally generated by oxidation with oxygen when the foil's surface is exposed to the air. However, the oxide layer may be chemically removed by soaking the foil with diluted nitric acid.
If you have growth then the ambient conditions are causing the deterioration. Most likely hydrated chloride is the culprit. Commercial "deox" formulations do a better job in oxide removal. The use of reducing agents in solution might also be possible, as well galvanic reduction coupling (where the Ni or Cu reduce as counter metals used oxidize) may be tried in an appropriate electrolyte. A real answer depends on the final use for your metals as well as the degree of corrosion caused by whatever the ambient conditions are. All metals have a layer of oxide, even Au.
already dilute HCl does the job, after this, you rinse well with DI water
I forgot about the old reduction reaction by the likes of cerium oxide. A paste of it rubbed onto surface both reduces oxide as it abrades the surface. There have been many polishes formulated with CeO or CeO2 in the past century.
you do cathodic cleaning with dilute HCl with an mercaptan as inhibitor
For Cu: Use dilute solution (10-20v%) of orthophosphoric acid (H3PO4) in water.
For Ni: Use methanol (~93v%), water (6v%), HCl (0.5v%), HNO3 (0.5v%) and FeCl3
I forgot the exact composition of this etchant. If you need that write back to this post. Although Ni form NiO immunity layer very fast. You have to preserve the clean Ni surface very fast.
We electrochemists are always in hunt of better etchant. Some thing better may be out there. The etchants I mentioned worked for me.
Answers always depend on the intended application query. If application is absent only speculation applies. So, ask the RIGHT question?
If all else fails foils then wet fire them. Ni foil will become inert and Cu will not necessarily (I don't know). See 'Nickel by Hydrogen'.
Also, delve into the preservation techniques used for archaeology finds.
Both copper(II) oxide and nickel(II) oxide layers formed over their respective metals can be dissolved with hot diluted acetic acid.
Did I mention that halides are oxidation catalysts? A little goes a long way. Chloride mechanistically transfers into a subsurface layer during catalytic oxidation action. In a related reaction here's a way to grow pure Copper in a chloride medium. Place Copper sulfate crystals onto a bed of salt in a glass vessel and add more salt for cover. Place a filter paper over to cover and finally, a plate of iron to weigh it all down. Pour brine in to fill it up and a cork float that helps insulate in heat overnight. Tear it down the next day to yield beautiful mirror-facets of hexagonal Copper crystals with 99.99+% purity and the gold color. Unless protected, Copper will oxid. in a few days.
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