Use the "sum out" outputs on the 4-channel amp to send the signal to the two channel amp. Don't bother with 5.1 or the orange sub output on the sound card, and set your sound card for two speaker stereo. The two channel amp should be set to LP, or low pass, and the four channel amp to either FR full range or HP high pass, but use HP only if you can adjust the crossover frequency. If your sub is <70Hz, then limiting the <70Hz frequencies from the front speakers may have some advantages. Most likely you'll want to use FR on the 4-channel.