We develop a geometric approach toward an interplay between a pair of quantum Schur algebras of arbitrary finite type. Then by Beilinson-Lusztig-MacPherson's stabilization procedure in the setting of partial flag varieties of type A (resp. type B/C), the Howe duality between a pair of quantum general linear groups (resp. a pair of ıquantum groups of type AIII/IV) is established. The Howe duality for quantum general linear groups has been provided via quantum coordinate algebras in [Z02]. We also generalize this algebraic approach to ıquantum groups of type AIII/IV, and prove that the quantum Howe duality derived from partial flag varieties coincides with the one constructed by quantum coordinate (co)algebras. Moreover, the explicit multiplicity-free decompositions for these Howe dualities are obtained. Contents 1. Introduction 1 2. General construction for arbitrary finite type 4 3. Howe duality for quantum general linear groups 9 4. ıHowe duality for ıquantum groups of type AIII/IV 16 5. Quantum coordinate coalgebras 23 6. Multiplicity-free decompositions of ıHowe dualities 28 References 32