You may need to brush up on the history of currency. So called "gold and silver" standards did not eliminate fluctuating exchange rates and valuation/devaluation cycles.

For example:

"In 1663, a new gold coinage was introduced based on the 22 carat fine guinea. Fixed in weight at 44½ to the troy pound from 1670, this coin's value varied considerably until 1717, when it was fixed at 21 shillings (21/-, 1.05 pounds). However, despite the efforts of Sir Isaac Newton, Master of the Mint, to reduce the guinea's value, this valuation overvalued gold relative to silver when compared to the valuations in other European countries"

http://en.wikipedia.org/wiki/Pound_sterling#Gold_standard

So even in physical coin form, this only worsened with introduction of paper notes, individual currencies fluctuated in valuation from country to country.

Thus, European currencies historically were not "the same".

There were fluctuations of gold relative to silver. There were not fluctuations of gold relative to gold, or of silver relative to silver.

You a projecting modern commodity trading exchange concepts onto history - until the advent of "virtual" trading of gold - there DEFINITELY was fluctuations of the value of gold and silver between countries.

But at the time, not all gold coins were the same. A florin from northern italy was not necessarily worth the same as a ducat of identical weight. Different mints formulated their metal differently and counterfeiting was a real and serious problem. The rumour that a king had ordered the adulteration of the currency could cause prices to swing wildly. Asset backed currencies were not some edenic pre-lapsarian device of peace and plenty. As an economic mechanism they have serious problems; in some respects more so than a fiat currency.

You've just criticized bimetalism, not the concept of a backed currency. Gold will be the same value relative to gold; it's just impossible to peg gold to silver.

You could also reasonably argue that the US is a perfect example of what the OP thinks is impossible. The primary differences are that we have a far more mobile workforce than does Europe, we have effective caps on state debt levels, and we have a (much hated) unified tax system that allows federal tax money to be spent in states that are lagging economically. Despite conservative hatred of these latter two items, they have done a lot to keep us from going through Greek debt crises of our own.

The GP meant not that it's physically impossible, but rather that it is a bad idea.

(I'd rather not get into an argument about the gold standard; except to say that almost every economist that matters thinks it is a bad idea.)