If you played New Vegas, there were 3 currencies. Caps, Legion denarius, and NCR monies.
And aside from Caps, they were completely useless. In fact if you look in the contruction set, Legion and NCR currencies really weren't currencies at all- just clutter with predefined (and universal- a non-Cap currency item worth 5 caps was worth that 5 caps in cap territory, Legion terrritory, or NCR territory.) values you could use in trade (like any
other clutter) or sell off for Caps- aka currency.
If we went to multiple currencies, I'd hope they were handled much better than this. We're in a civil war- why wouldn't the faction wanting to throw the Empire out have their own currency, just like the Confederate States tried during the US Civil War? Then add in gameplay-affecting details: hardcoe independence merchants could refuse (or severely devalue) Septims (Or Medes, or whatever the current Empire is calling their coins) or flat-out refuse to trade in anything but their own local Skyrim currency.
tl;dr: If we have multiple currencies, make them real currencies...not just some clutter that looks like currency in order to fake it.