Yep, there is a very nice sidewalk that connects the two! The sidewalk is along the main road that is outside BB and CSR. I used the sidewalk once to go from CSR to BB - this is probably not recommended, but I rented a bike at CSR, and rode it over to the entrance of BB (but did not go in). You can definately walk it, though it may be a bit of a hike. If you been walking alot in the parks, it may be more than you want to do, though!
(As for riding a bike from CSR to BB, I have no idea if Disney allows guests to do that, but no one ever said anything or stopped me!

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