The prophet ﷺ did not distinguish between the two surahs until the words “In the name of Allah, the Compassionate, the merciful” was revealed to him. These are the words of Ibn al-sarh.
كَانَ النَّبِيُّ ﷺ لاَ يَعْرِفُ فَصْلَ السُّورَةِ حَتَّى تُنَزَّلَ عَلَيْهِ { بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ } وَهَذَا لَفْظُ ابْنِ السَّرْحِ