Here's a good article on that engine. And no, it's NOT two V6s bolted together. Has it's own block and crank. The heads are shared with the V-6, as well as exhaust manifolds and maybe intakes, thought the intakes I've seen were unique to the V-12. Most dimensions, such as bore centers, bore, V angle (60*), etc. are the same so that many parts are interchangeable. The V-12 is based on the V-6 design, it's just not two V-6s bolted together.
There have been quasi V-12s and V-16s made that way, and I've even seen a quasi I-12 -- those are just two engines couple together driving a single transmission and timed together, but no manufacturer that I know of ever produced such a setup for a production vehicle.