We know that when there is a case of someone that is obligated to perform a mitzvah m'drabbanan (and in fact there are 2 d'rabbanans together) they can't be "motzei" someone in the mitzvah who only has a chiyuv because of 1 d'rabbanan.
Here are some examples of this:
The Mishneh Brurah in Siman 186 Sif-Katan 7 writes that a katan that didn't eat to the point of satiation there are some poskim that hold he may not be motzei his Father who also didn't eat to the point of satiation because his Father has a obligation to say bircas hamazon because of 1 d'rabbanan and his son has an obligation because of 2 d'rabbanans.
Another example is in the Sharei Tzion Siman 271 Sif-Katan 2. If a person davened already friday night then they are obligated in Kiddush m'drabbanan. In such a case the katan could be motzee the gadol since he is also chiyuv m'drabbanan (See there M"B Sif-Katan 2.) However the Sharei Tzion (b'shem the Derech HaChaim) that if the katan already davened he couldn't be motzei the gadol since the katan is 2 d'rabbanans and the gadol is one.
Another example is in Mishneh Brurah Siman 689 Sif-Katan 6. If someone hears from a katan the megillah they are not yotzee. The M"B there explains that even though the chiyuv of a katan by megillah is d'rabbnanan (just like the gadol) still there is an additional d'rabbanan in regards to the fact that the obligation on the katan in general in all mitzvos is d'rabbanan.
I had assumed as well that another example would be kiddush by day. Since for a gadol it's d'rabbanan (one) and for a katan it's two (the kiddush itself together with the general chiyuv (however I don't have a written source for it, I think the above examples are enough for now.
The question is, do we also say the one who has 3 d'rabbanans can't be motzei the one that has 2? (An example would be kiddush by day on yom tov sheini shel golius. The katan has a obligation because of 3 d'rabbanans and the gadol's obligation is because of 2.)