Which Alpine cd changer do you have?? Depending on what model it is, you could buya corresponding Alpine head unit that connects directly to the changer for the best sounding results. If you want to keep everything stock looking, then you need to make sure your Alpince changer either can plug into your deck,if its the right one via a special cable, or it has a FM modulator to play through a radio station on your factory deck.

Having 2 rca's givies you more flexibility in regards to adding amps. 1 pretty much limits you to one amp unless your amp has an rca output. You could also add an EQ or crossover that would allow for more rca outputs.

The item you have linked at Crutchfield is what you would use to obtain rca outputs if you stayed with your factory deck. Hopefully, you also know that you'll need an amp kit to hook up any amps you add to power your JL sub.