They summoned the goddess Yogamaya and experienced her transfer the fetus from the womb of Devaki on 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 child conceived in a single womb https://ewr1.vultrobjects.com/blissdrive/SouthernCaliforniaSurrogacy/Becoming-A-Surrogate-In-Southern-California.html