There is a tradition of calling out the name of Radha and Shyam together because it is believed that without Radha Shyam is incomplete. Perhaps this is the reason why this mythological coincidence has exists that while Krishna Janmashtami is celebrated on the Ashtami date of Krishna Paksha of Bhadrapada month, Radhashtami is celebrated on the Ashtami date of Shukla Paksha.