I would recommend to perform a cultural due diligence to assess if the value of the two football clubs are aligned or if a a new common culture should be created. The football club culture also depends on the supporters.
In parallel, I would also recommend a sales/marketing due diligence to assess the potential related to: ticket sales, derivatives products, TV rights, prize money and other events.
Finally, I would perform an anti bribery anti corruption due diligence in order to make sure that the buyer is not exposed to successor liability given that soccer player transfers often (sadly) made the front page of the news paper for the bribes involved