Ji yu Ippon Kumite is one step free sparring. In this exercise the attack is limited to one move, but the defender does not know what that one move will be. This is not a pre-set partner work but limiting it to only one attack frees the defender from the worry of being hit with multiple follow up techniques if their defence does not work. The exercise can be expanded to two attacks (Ji Yu Nihon Kumite) or three attacks (Ji Yu Sanbon Kumite) etc.. However, it should be noted that pure Wado Ryu karate attempts to defeat an opponent by redirecting their attack. In essence this should mean the defender should be able to stem the attack on the first technique. If your first defence allows the attacker to follow up with a second attack it is not really an effective defence. Having said that of course, it is very rare that I actually manage this!
Ji Yu Kumite is "Free Sparring". There is no pre-set attack or defence and no limit to how many techniques can be performed concurrently.