Is the 'aqiqah obligatory for a father who is in debt, yet able to provide it with effort, and is it permissible for him to perform the 'aqiqah for twin girls with one sheep or two?

The majority of jurists hold that the aqiqah is a Sunnah (recommended act), not obligatory, and it is not binding on you, especially if you are unable to afford it. It is, however, desirable to borrow money for the aqiqah, even if you are in financial straits. The prescribed sacrifice is one sheep for a girl and two equivalent sheep for a boy.