A post on devforums clarifies that tvOS apps will be standalone. You will not be able to build a universal app that works on both iOS and tvOS. However, you will be able to share code — assuming I can ever get my conditional compilation of
#if os(tvOS) working.
Don’t confuse universal apps with the universal purchases mentioned in the write-ups: “Empower customers to enjoy their favorite apps on both iOS and the new Apple TV with a single purchase by enabling universal purchase for your app on the App Store.”
It looks like you’ll be able to create a bundle that sells with components for both systems, even if there are two actual apps that you’re building.