2.2.32 द्वन्द्वे घि

घि संज्ञक is used first in the case of द्वन्द्व. For example, हरिश्च हरश्च - हरिहरौ (since हरि is  घि संज्ञक). However, सखि is not घि संज्ञक - hence सुतसखायौ, सखिसुतौ.