I think manufacturers also link brakes because a lot of riders tend to just use the front brake during hard/sudden braking. It might do most of the work, but not all. That being said, I'm with Monte on keeping them separate. I have bad dreams about blowing out a front tire and not being able to only use the rear brake as I come to a stop.
Jeff, I hate going to NAPA, but they are the best when it comes to getting hoses/fittings and such... while some of the other stores just look at you like a deer in the headlights with "year, make model?" Honestly, if you were to ask them for a water pump for a Ford small block, they'd ask "year, make, model?"
And Georges, brake hoses sold by auto parts stores must meet DOT standards. One big reason the BMW hoses (or most motorcycle parts, for that matter) are so expensive is because motorcycle parts are produced at a fraction of the volume of auto parts. In fact, some of the really nice braided stainless lines are not DOT certified... and don't doubt that insurance company lawyers would try to discredit the performance of your smashed up machine should they be standing opposite you in court.