In many WLANs scenarios, the load transmitted from the AP to the clients (Downlink), far outweighs traffic demand from the clients to the AP (Uplink), thereby yielding traffic asymmetry. Moreover, when many clients associate with a single AP, the clients can cause a disproportional amount of medium contention compared to the AP, producing contention asymmetry. We present Dual Wi-Fi, a MAC architecture and protocol that minimizes MAC overhead by matching spectrum resources to traffic asymmetry. Dual Wi-Fi separates uplink and downlink data traffic into two variable-width independent channels, each allocated in accordance to the network's traffic demand. Our experimental and simulation results demonstrate that Dual Wi-Fi matches downlink vs. uplink throughput ratio to demand ratio within 1% under any client density and traffic load. Through this matching capability, Dual Wi-Fi offers unbounded downlink gains as congestion increases, minimizing and in some cases eliminating retransmissions and contention time.