Tanto monta

Tanto monta
 

Hi,

Tanto monta, monta tanto, Isabel como Fernando.

This is a motto described in Wiki and elsewhere, so this isn't a question about its meaning. What would be the literal translation?

It could be something like
"Isabel is worth as much as Fernando"

Thanks! Is it a cast iron idiom, or the meaning can be traced to the etymology of the word montar? They ride on equal terms, or something?

This is a saying alluding the Catholic Monarchs of Spain, Queen Isabella and King Ferdinand.
As I understand, King Ferdinand used the Gordian knot as an emblem. This is the knot that Alexander the Great cut with his sword, supposedly expressing the motto: "Tanto monta cortar como desatar" ("It is as valuable to cut as to untie"). The Catholic King associated his device to the abbreviated motto: "Tanto monta".
It is known that Isabella and Ferdinand exerted power equally, so "tanto monta, monta tanto..." was a burlesque version of the original motto that got popularized because of the rhythm introduced by "mirroring" the motto (which has the same amount of syllables as both proper names), and the rhyme by introducing the name of the king at the end.
Also, in this last version "montar" can be interpreted as both "to be worth" and "to ride horses", which is something women weren't supposed to do, but Isabella would.

It rang the bell, finally. What about the English word tantamount? :p

The English word tantamount, as etymology has it, means 'to amount to as much' or 'to have equal value'. Our word comes from Anglo-French tant amunter (or Italian tanto montare).