It is not that Rādhārāṇī is separate from Kṛṣṇa. Rādhārāṇī is also Kṛṣṇa, for there is no difference between the energy and the energetic. Without energy, there is no meaning to the energetic, and without the energetic, there is no energy. Similarly, without Rādhā there is no meaning to Kṛṣṇa, and without Kṛṣṇa there is no meaning to Rādhā. Because of this, the Vaiṣṇava philosophy first of all pays obeisances to and worships the internal pleasure potency of the Supreme Lord. Thus the Lord and His potency are always referred to as Rādhā-Kṛṣṇa. Similarly, those who worship Nārāyaṇa first of all utter the name of Lakṣmī, as Lakṣmī-Nārāyaṇa. Similarly, those who worship Lord Rāma first of all utter the name of Sītā. In any case—Sītā-Rāma, Rādhā-Kṛṣṇa, Lakṣmī-Nārāyaṇa—the potency always comes first.
Rādhā and Kṛṣṇa are one, and when Kṛṣṇa desires to enjoy pleasure, He manifests Himself as Rādhārāṇī. The spiritual exchange of love between Rādhā and Kṛṣṇa is the actual display of Kṛṣṇa's internal pleasure potency. Although we speak of "when" Kṛṣṇa desires, just when He did desire we cannot say. We only speak in this way because in conditioned life we take it that everything has a beginning; however, in spiritual life everything is absolute, and so there is neither beginning nor end. Yet in order to understand that Rādhā and Kṛṣṇa are one and that They also become divided, the question "When?" automatically comes to mind. When Kṛṣṇa desired to enjoy His pleasure potency, He manifested Himself in the separate form of Rādhārāṇī, and when He wanted to understand Himself through the agency of Rādhā, He united with Rādhārāṇī, and that unification is called Lord Caitanya. This is all explained by Śrīla Kṛṣṇadāsa Kavirāja in the fifth verse of the Caitanya-caritāmṛta.