Some early Hebrew manuscripts have the text of Psalms 114 and 115 together, combined as one. This practice was picked up by early Greek translations of the bible like the eptuagint (early 3rd century BCE), as well as by Jerome in the Latin Vulgate (late 4th century). Later manuscripts separated them, and pre-Reformation scholars have treated them as two distinct psalms, because of perceived differences in tone. One argument against separating them that held through the middle ages was that Psalm 114 lacked some sort of formal liturgical ending. Psalm 113 ends with "Praise the Lord!" as does Psalm 115. If you were an editor, what would you decide?