sei krsna--navadvipe sri-caitanya-candra
sei balarama--sange sri-nityananda
sei krsna—that original Krsna; navadvipe—at Navadvipa; sri-caitanya-candra—Lord Sri Caitanya Mahaprabhu; sei balarama—that Lord Balarama; sange—with Him; sri-nityananda—Lord Nityananda.
That original Lord Krsna appeared in Navadvipa as Lord Caitanya, and Balarama appeared with Him as Lord Nityananda.
Link to this page: https://prabhupadabooks.com/cc/adi/5/6