The coexistence problem of different wireless protocols that share a common frequency spectrum has attracted much attention owing to the proliferation of heterogeneous wireless networks in the research community. Recently, Bluetooth (BT) and Wireless LAN (WLAN) protocol stacks have been integrated as a single-chip communication module, and they now even share the antenna as well as the spectrum. In this paper, we show that this integration provides new opportunity for one protocol to better understand the other and to operate in harmony to avoid mutual interference. We develop an Opportunistic Bluetooth Transmission (OBT) scheme that enables a dual stack device having an integrated module to exploit previously-unused deferring times of the WLAN protocol. We evaluate its performance through not only model-based analysis but also practical implementation in a prototype testbed. The results show that the OBT scheme can significantly improve throughput of the dual-stack device.