Valmiki Ramayana Bala Kanda Sarga 4 Shloka 18
Original Shlokaप्रविश्य तावुभौ सुष्ठु भावं सम्यगगायताम्। सहितौ मधुरं रक्तं सम्पन्नं स्वरसम्पदा।।1.4.18।।
1 source checked for this shloka
तौ उभौ both of them, सहितौ in unison, भावम् in theme, सुष्ठु excellently, प्रविश्य having entered, मधुरम् melodiusly, रक्तम् entrancing, स्वरसम्पदा sadja and other notes, सम्पन्नम् endowed with, सम्यक् very well, अगायताम् chanted.
They both chanted in perfect unison in theme and passion, with their charming melodious voice endowed with the rich notes of music (sadja and other).