They summoned the goddess Yogamaya and experienced her transfer the fetus through the womb of Devaki towards the womb of Rohini (Vasudeva's other spouse who lived along with her sister Yashoda throughout the river Yamuna, during the village of cowherds at Gokulam). Hence the kid conceived in a single womb https://digitalasset.b-cdn.net/SouthernCaliforniaSurrogacy/Becoming-A-Surrogate-In-Southern-California.html