3 channels into a 2 channel amp

[RG501]

what would happen if he simpily split the wire going from one channel?

[dice daniels]

Are you saying hook both 6x9's up to say the left and the sub up to the right?

If he did that he would need to run a low pass crossover between the deck and amp on just the right chanel and run the amp in full range. that way the sub gets nothing but lows and the 6x9's get fullrange sound.