If it's the same as on my CTK-5000, then this feature appears to only measure reaction times, no matter which hand you use. IOW, no need to configure for either or both hands. Use it as-is for either or both, just ignore the highlighted finger-graphic and use the finger-number instead. Put some tape over the hand-diagram if that'll help. If using it for both hands, then maybe choose an arbitrary key as the dividing line as to which hand will be used, or not. Though I don't find this feature to be particularly useful. It only trains your mind to respond faster to the built-in hand-diagram than any actual music. It's more of a reaction-time game than any kind of useful lesson. At least that's what it seems to be, to me.
I tried seeing if pressing the Accompaniment button would allow you to select another hand or hands for it, as it does for lesson songs; but the manual says it locks out all other buttons but power, start/stop, and music-challenge when this game is engaged.