Shulchan Arukh, Orach Chayim 105/שולחן ערוך·אורח חיים ק״ה

1The Law Of One Who Prays Two [Amidah] Prayers. Containing 1 Se'if: One who prays two [Amidah] prayers, one after the other, must wait between one and the other [for the time it takes] to walk 4 amot, so that one's understanding may be settled, [in order] to pray with the language of supplication.