From Academic Kids
Double majority is used in the United States for some votes on issues such as a tax levy or bond. This vote requires both a plurality of votes cast, and a majority of registered voters to cast votes in the election. This mechanism is used to prevent a small group from passing spending measures which affect the entire population in order to support their pet causes, especially at an election expected to have low voter turnout.
In the European Union, double majority voting is a form of Qualified Majority Voting which is proposed in the draft Treaty establishing a constitution for Europe. According to this proposal, any decision taken under this scheme will require the support of at least 55% of the Council of the European Union members who must also represent at least 65% of the EU's citizens.
In Australia, constitutional referenda require a majority of votes (51%) in a majority of states (4 out of 6). The two federal territories are now counted in the majority of votes, but not being states, they are not counted in the majority of states.