Playing two Player Games on One Player Cabinet

[GregD]

Hey there.  I searched for thr answer but can't find anything anywhere.  I decided to build another machine after something like 13 years and this time I am building a Raspberry Pi-based vertical one player bartop.  I want to keep it small so i am only doing one player controls.  I am using a Zero Delay USB encoder and I am using the Mr. Burns 900 in 1 image, at least to get it up and running.  What I want to do is have both one and two player buttons on the machine.  When I am in a two player game, hit the Player 2 button and then have the two players alternate using the one set of controls.  Any idea how I can do this?  Do I need another encoder?  Is the solution within the Retropi settings or is there something int he emulator I need to change?  The answer is probably very simple but I am not seeing it.  Thanks