Always check two cable systems for inadequate slack.
Everything else can be perfect, but if there is not enough slack, it will bind.
A quick test is to make sure the throttle bodies or butterflies are truly closed and resting against their stops.
Then attempt to twist the throttle backwards.
If it won't move in the reverse direction with fairly light pressure, then suspect inadequate slack.
It is so easy to get bitten by this when cables get changed, because of the amount of jam nut movement one typically does to allow easy removal of the old cable and easy fitting of the new.