if you are playing from the PSN store then just press the PS button and select the next disc.
if you are playing from a converted PSX game (as in 1 eboot per disc) then just save, open the next disc eboot and load the save data (this is how I orignally played FFVII).
I always recommend downloading PSN-PSX games though, they are just overall easier to use and you can customize the controls. Just be sure to download both the game eboot & the activation file (can't remember the actual name, but you need it to trick the PSP into thinking you bought the game).