Does an expiation (fasting for two consecutive months or feeding sixty poor people) become obligatory upon one who swore by a religion other than Islam, then committed the sin that he swore not to commit with the intention of disbelief, then returning to Islam, or is merely uttering the two testimonies sufficient for his Islam to be accepted?

Swearing by disbelief (kufr) is forbidden. The general principle is that it does not lead to disbelief unless one intends apostasy from Islam by it. The expiation for an oath (kaffarat al-yamin) is not obligatory in this case, according to the majority of scholars. However, if one performs it as a precaution, its expiation is feeding ten poor persons, or clothing them, or freeing a slave. If one is unable to do so, then it is fasting for three days. There is no fasting for two months in this situation. The repentance of an apostate is by uttering the two testimonies of faith (Shahadatayn). If their disbelief was due to denying an obligatory act, then their repentance, along with the Shahadatayn, involves affirming that which they denied.