player switch for a O and X game

I've created a basic game of noughts and crosses using buttons as an aid to learn programming.

I want to add a feature so players can select O or X for each player and use it to indicate if player one is x and player two is o or visa versa. I tryed to use radio buttons but couldn't have two options on at the same time. I also tried check boxes . Was wondering if there is a better way cheers