What does E.E.U.U. stand for?

Factual Questions
1

I was watching the World Cup on the Spanish Channel. For the U.S., they use E.E.U.U. as the abbreviation. What does it stand for?

2

Apparently because both Estados and Unidos are plurals, the abbreviated version has two letters for each (the site below compares it to using pp. for ‘pages’ in English):

Why is the abbreviation for Estados Unidos written EE. UU. instead of simply E.U.?