The double fathah, kasrah, and dammah signs are not merely graphic marks. During connected reading (wasl), they contribute an n-like ending and its relation with the next pronounced consonant determines a Tajwid ruling.
«عَلِيمٌ حَكِيمٌ»
All-knowing, all-wise. Ha after tanwin requires throat clarity.
«سَمِيعٌ بَصِيرٌ»
All-hearing, all-seeing. Ba after tanwin requires conversion.
At a pause (waqf), the final pronunciation may change and the following word is not joined. Therefore, the connected-reading rule of assimilation or concealment does not automatically apply. Always determine whether the phrase is being joined or stopped.