For the visarga of words ending in is and us before a hard guttural or a labial, there is optionally substituted ṣ when the two words stand in correlation with one another.,
The ष is understood here. Thus सर्पिष्करोति or सर्पिः करोति, यजुः करोति or यजुष्करोति ॥
Why do we say 'when the two words are correlated'? Observe तिष्ठतु सर्पिः, पिब त्वमुदकम्, where सर्पिः is not in construction with पिब, but with तिष्ठतु ॥
The word सामर्थ्ये here means व्यपेक्षा or mutual relation of two words; and not \having the same meaning\, or it may mean both. In fact सामर्थ्य is equivalent to आकाङ्क्षा i.e. the syntactical want of another word to complete the sense. It does not here mean 'compound'. For it being a पदविधिः the word समर्थः is understood here (समर्थः पदविधिः (II. I. 1)). The employment of the word सामर्थ्य here indicates that it is a different samarthya from that of 2.1.1. It does not denote एकार्थीभावः or ऐकार्थ्य which is the samarthya of compounds where two or more words denote one object. The samarthya here means vyapeksha, which is thus defined नानाभूतयोः, पदार्थयो र्यौ शब्दौ, तयो र्यो योगः \the syntactical union of two words expressing two different ideas\.,