The gemorra tells us on 4b (as in other places in shas) that we do not make a gezeira on a gezeira. But in the very next sugya on 5a we are informed of (what seems to me as) a gezeira on a gezeira of "shema yifchat" (the entire issur of carrying in a mavoy is a geizera d'rabbanan so our concern "shema yifchat" is a trei d'rabbanan.
"Shema Yifchas" is not a Gezeirah l'Gezeirah. In the case of a Mavoy, the Rabanan instituted a way of amending the Mavoy so that one may carry in it l'Chat'chilah (that is, erecting a Korah). When they originally made their enactment, they included the laws and guidelines for making a valid Korah. Thus, the rule that the area beneath the Korah that decreases the height to within twenty Amos must be at least four Tefachim wide, according to Abaye, was stated at the same time that the enactment permitting carrying in a Mavoy was made, and thus it is all part of one enactment. (I.e., "Kula Chada Gezeirah Hi")
Even though the term " Gezeirah Shema Yifchas" implies that it was a separate decree, following the original decree of making a Korah in the first place, the Gemara actually means " Chashash Shema Yifchas." That is, concern that the area might become diminished was part and parcel of the original enactment of Korah.