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When an IEEE 802.11 ad hoc network achieves a capacity C using a single channel, the targeted capacity using two channels should be 2 · C. We believe that this should be used as the benchmark for capacity comparisons for multichannel schemes. However, most of the dual-channel 802.11 protocols proposed in the literature appear to achieve less than 60% of the 2 · C targeted capacity. In previous work, we proposed a link-directionality-based dual-channel medium-access-control (MAC) protocol (DCP) to boost the network capacities by up to 78% of our targeted capacities, i.e., 78% x 2 · C = 1.56 · C. However, the DCP still fails to reach the 2 · C capacity target due to the overheads incurred by the protocol. In this paper, we implement a signal-to-interference ratio (SIR) comparison algorithm (SCA) on top of the DCP in an attempt to further improve network capacity. This algorithm incurs relatively small overheads and can further relax the protocol constraints that are imposed by the virtual carrier-sensing mechanism. Interestingly, while the capacity of the pure DCP decreases when link lengths are short, the capacity of the pure SCA increases when link lengths are short. The two algorithms compensate for the downside of each other to bring about a more uniform capacity improvement. Simulations show that the proposed scheme [described as DCP with SCA (DCPwSCA)] can increase the network throughputs by 430% in lattice topologies and 213% in random topologies. [ABSTRACT FROM AUTHOR]