Even if your software would handle which app uses which NIC, the chances are both NICs are attached to the same access point thus rendering the exercise moot. Remember wireless is simplex communication. When anyone or anything is using the channel, you cannot send or receive. Even if it is your neighbor, and lord forbid he still has a 802.11b client. If your access point can see any 802.11b client then you will be running in RTS/CTS protection mode and your wireless will not be running as efficient as possible.
Turn off your lower data rates. These rates are not needed around a normal house. Ideally you want 11mbs and below disabled with 12mb being mandatory with all others supported. This will result in a performance increase if there are any 802.11b clients in the area (remember not just your devices but anything within RF range of the access point). In addition, the 12mb beacon rate is much faster than the 1mb beacon rate which means less channel overhead and more time for client frames.
If you find turning off these data rates causes you issues, then i would suggest another access point to cover the area having issues or in a pinch you could get larger antennas. Larger antenna presents other problems like hidden-node scenarios and creates unbalanced cells. Hidden-Node can cause the same RTS/CTS protection mechanisms used for 802.11b clients to be enabled causing performance issues. You avoid hidden nodes by managing your gain. You want your overall power to be relatively close to those of the clients. This allows clients on the access point to see each other and avoid issues.
To avoid issues with wireless you must think in terms of wireless quality vs quantity. What i mean by this is you really need to sacrifice coverage for quality. To make up for the coverage you add access points.