The currency tag should be used for such questions as:
Politics of placing an image on currency, or
Revaluation of currency (exchange rates), or
The use of a particular currency for international trade.
The money tag should be used for such questions as:
The only time the two tags should appear together would be for a question concerning the cost of changing or the revaluation of currency.
When viewed this way, they are not synonyms; though, I think the tag info for the currency tag could be improved to highlight the distinction.
From Personal Finance & Money SE,
What is the difference between money and currency?
Is currency a subset of money?
You can talk about money without referencing notes or coins, but you can't talk about currency without referencing them.