Technically, it is almost impossible and often prohibited for a flight attendant to work for two different commercial airlines at the same time. Most airlines have strict "non-compete" clauses in their contracts that mandate exclusivity; working for a competitor is usually grounds for immediate termination. Logistically, it would be a nightmare because flight attendants are on "reserve" or fixed schedules that require them to be available on short notice. Furthermore, every airline has different safety protocols and FAA certifications for their specific aircraft; holding two sets of active certifications simultaneously would lead to critical safety confusion. The only exception is occasionally seen in the "private" or corporate aviation world, where some flight attendants work as contractors for multiple private jet owners, but even then, they are not typically working for "different airlines" in the commercial sense.