After manas when the compound is a Name, the Instrumental -3rd case endings are not elided before the second member.,
Thus मनसादत्ता, मनसागुप्ता, मनसासंगता ॥ Why do we say 'when a Name'? Observe मनोदत्ता, मनोगुप्ता ॥,
मनसः सञ्ज्ञायाम्
Adhyāya 6 · Pāda 3 · Rule 4