I think the scene you are referring to is the 'Good Ending' of X-2, which you get by hitting X until you here 4 whistles in the farplane when yuna fal in and meets shuyin. Then after beating shuying while in the farplane glen you must hit X until you here the whistle again, a fayth will appear, select the first option, then after the normal ending you get an extra FMV.