Mobile gadgets including smartphones and laptops nowadays support both Wi-Fi and Bluetooth together. The two protocols coexist in the 2.4GHz ISM band while sharing a single antenna in order to meet constraints on form-factor. We call this kind of devices "Dual- stack devices" in this paper. Since Wi-Fi and Bluetooth on dual-stack devices work in TDM (Time Division Multiplexing) manner, the performance of Wi- Fi, specifically throughput, degrades inevitably. We propose a novel scheme, Opportunistic Antenna Utilization (OAU), which aims to improve the throughput of Wi-Fi by utilizing time assigned to yet not used for a Bluetooth streaming service. We first present measurement results showing the degradation of Wi-Fi throughput on dual-stack devices followed by a simple analysis to anticipate the expected gain of OAU. Then we verify our analysis by showing that simulation results coincide well with the analysis.