No, this is not true. Both gold and silver exist in a limited supply on the planet Earth, and both can serve as money. There is no reason why any number of finite supply *coin block chains can not simultaneously exist.
By that logic there is no reason why all matter can't be currency. Lots of rarer harder to extract materials than gold or silver.
So just as you can have:
Bitcoin
ScamCoin
CrapCoin
JunkCoin
StupidCoin
MyGodArePeopleFallingForItCoin
You can also have:
Gold Coins
Silver Coins
Platnum Coins
Palladium Coins
Helium Coins (He3)
Xenon Coins (Xe126)
....
287 other coins (even excluding radioactive isotopes)
...
Carbon Coins
However just like with Gold it is likely one currency will remain the most desirable and retain the most value/utility. People are crazy if they think in the future some merchant is going to accept 498439048239403849023 different coins. Hell getting them to accept Bitcoin is difficult (just like getting a merchant to take a Gold coin). Expecting them to accept ScamCoin or Xenon Coins is just naive.
You give merchants very little credit.
Many companies are already multi-nationals operating in multiple currencies, a lot of tourist shops in Europe and even some in the USA accept Euros, Dollars, Pounds + other local currencies. Multi-currency Cash registers are relatively commonplace in airports and accross continental Europe. Most people "get" the concept of multiple currencies and exchange rates and have no problem at all adopting them.
I agree that accepting CryptoCurrency *at all* is a big jump to make for an online retailer, and an even bigger jump for a bricks-and-mortar shop, but once one is in, all can be in.
Back before Fiat, Gold & Silver were two separate metals being widely used worldwide as currency, with various other metals trading around the periphery, Copper, Lead, Tin etc. so I think at the very least there is room for 2 "metals" in the market place, probably more.
It's wrong to say any physical matter can be used as a currency, a coin made from sodium or sulphur would be ridiculously impractical and last about an hour.
Gold and Silver have a number of unique properties which make them perfect for use as currency. A few other metals such as Platinum and Palladium also fit the bill, but realistically you are looking at maybe 8 or 10 metals that could realistically be used as currency.