By John O'Donnell and Alastair Macdonald FRANKFURT/ BRUSSELS (Reuters) - The thinning oxygen supply to Greek banks could leave Athens with little choice but to introduce a form of second currency, European officials believe, a temporary means of freezing its membership of the bloc but one that risks its ultimate exit. Facing a massive budget shortfall, California's then governor Arnold Schwarzenegger issued more than 300,000 IOUs, known as warrants, with a value of almost $2 billion, to pay taxpayer refunds and others due money from the state.