In kihon Ippon, after you have attacked, and your opponent has blocked, he/she then has (what Gwyn calls) a "free shot" i.e. they counter without you blocking or reacting. However, it is considered very bad form to connect with the counter. Mistakes can happen, but the intent should not be to make any significant contact. As we progress through the different kumite drills, then we get to drills in which you are reacting e.g. nan Demo Sanbon. The "rules" are then different.
Combination counters are sometimes used by Dan grades, kyu grades should - by and large - stick to the single counter of Kihon ippon, as this develops control.
Lloydie