It is possible that this means one rak'ah with the imam and the other rak'ah prayed alone, because the obligatory prayer remains as it originally is.
Imam Ahmad (may Allah have mercy on him) and other hadith scholars hold that acting upon every form of the fear prayer that has come in the hadiths is permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخوف / 6055