Also a reminder that arcade games commonly shared a single set of controls, for example pacman. So player 2 actually uses player 1 inputs when it's their turn. There were also games that had distinct player controls such as tmnt or mortal kombat. It would help to specify the game your trying to play to determine if that plays any part in your issue.
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first