The nativity account names the two resins together but does not describe them blended or burned as one compound, and a genuine combined incense recipe naming both resins together has not been located.
What would resolve this A study of ancient or early Christian liturgical incense recipes identifying the earliest instance in which frankincense and myrrh are compounded into a single burned mixture, rather than named or given as a symbolic pair.
OpenLiturgical history, history of pharmacyMircea Eliade (ed.); Lindsay Jones (ed., 2nd ed.), Encyclopedia of Religion