As per Shulchan Aruch OC 301:13, one is not allowed to wear/carry a garment whose sole purpose is to protect other clothing that he is wearing ("אצולי טינוף"), though if those also protect the wearer then it's allowed.
Ramma (301:36) writes that one shouldn't wear two belts, one on top of the other, because if one only needs a single belt, the other belt is a "משאוי," a burden, and cannot be worn on Shabbos.
Mishna Berura writes (301:134) that women who wear a belt over a too-long robe, and then hike up the belt and put on another belt so as to prevent the robe from dragging on the ground does not constitute a problem of two belts even according to those opinions who generally forbid two belts, because both belts have a purpose: one to hold the robe closed, and the other to prevent the robe from getting dirty, and therefore neither belt is a משאוי.
כתבו האחרונים דמה שנוהגות קצת נשים לחגור בחגורה, ועוד חוגרים בחגורה על הבגד הרחב מלמטה, ועל ידי כך הבגד הגבה בגד הרחב מן הארץ, ועושין כן שלא יתלכלך הבגד בעת שיש טיט ורפש, גם כן שרי, כיון שצרך הוא לכך
Why isn't the belt that holds the robe up, preventing the robe from getting dirty, forbidden as "אצולי טינוף" like the items listed in SA 301:13?