Raakhee Venugopal’s answer
By common universe, I presume you are referring to the society in which both the Mahabharata and the Ramayana are set in. My answer is based on this presumption.
During the time period of both the Ramayana and the Mahabharata, Aryavarta or the Indian Subcontinent basically had the same kingdoms and ruling clans, though the seats of power had shifted with changing times.
The two maps above show Bharatavarsha or Aryavarta during the times of the Ramayana and Mahabharata, respectively. It can be seen that the major kingdoms still remained the same. As these societies were predominantly Vedic in nature, they followed similar customs and traditions. And strictly speaking, Ramayana and all its associated stories are narrated to the Pandavas as being a part of the history of Aryavarta, during their 13 year exile period. So it’s only natural that both epics shared the same societies.
I hope this answers your query.