The manipulation of micro-objects and fluids inside a microchannel using bulk acoustic waves (BAW) has been widely used in various applications. Although the physics of ultrasonics resonance modes in a fluid are well studied, the half wave resonance condition in the case of coflowing immiscible fluids has not received much attention. In immiscible coflowing fluids, there is a sudden jump in acoustic properties across the interface which alter the resonance condition. Here, using an analytical model we show that resonance can be attained by actuating individual fluids at a frequency proportional to the speed of sound in the corresponding fluid. Further, simulations are performed to obtain the half-wave resonance by actuating both fluids at a single resonating frequency that depends on the speed of sound, densities, and width of the individual fluid. In a coflow system with equal speeds of sound and densities, the resonating frequency is independent of the stream width. We show that a pressure nodal plane can be realized at the channel center by operating at a half-wave resonating frequency. The results of the model and simulations are verified experimentally via acoustic focusing of microparticles suggesting the formation of pressure nodal plane and hence resonance condition. When the coflowing fluids are actuated with higher acoustic energy at the resonance condition, the higher impedance fluid (HIF) relocates towards the center of the microchannel while the lower impedance fluid (LIF) moves towards the side walls. We present a theoretical formulation for the acoustic radiation pressure acting on the interface between the fluids, revealing that the acoustic radiation pressure is a nonlinear function of the impedance contrast. Relocation of different combinations of coflowing immiscible fluids is demonstrated through simulations and experiments. We find that the competition between acoustic radiation force and the interfacial tension underpins the relocation phenomenon. Our study shows that although the acoustic radiation force is directed from HIF to LIF, the final configuration after relocation mainly depends on the wall-fluid contact angle. The time evolution of the interface during the relocation process is studied and explained using the time variation of the pressure fields during relocation. Our study will find relevance in acousto-microfluidics involving immiscible coflow systems.