In a mantra , the suṭ is added to candra when it is second member in a compound and is preceded by a short vowel.,
Thus सुश्चन्द्रो युष्मान् ॥ Why do we say after a short vowel? Observe सूर्याचन्द्रमसाविव ॥ Why do we say 'in a Mantra'? Observe, सुचन्द्रा पौर्णमासी ॥ The उत्तरपद can only be in a compound (samasa) as it is well-known to all and it does not mean, 'the second word', as the literal meaning might convey : Therefore the rule does not apply here शुक्रमसि चन्द्रमसि ॥,