In a dvandva compound, that word-form which has fewer vowels, is to be placed first.,
Thus प्लक्ष +न्यग्रीध = प्लक्षन्यग्रोधौ ; and धबखदिरपलाशाः ॥
When there are many words, there is no fixed rule. As शंखदुन्दुभिवीणाः or वीणाशंखदुन्दुभयः.,
अल्प-अच्-तरम्
Adhyāya 2 · Pāda 2 · Rule 34