That is not at all how this works. This is 2015 level of misunderstanding. Two parties establish a channel, but they do not "trade" between each other. They utilize rhe channel liquidity to send and receive bitcoin across the network; not just between themselves. Alice and Bob open a channel, because they find it advantageous to route payments over eqch others nodes. Alice finds that Bob's location on the network graph establishes routes that increase probability of timely and cheap delivery. And vice versa.