Hmm...
I still have never heard it called that. I guess they have different names for it in Europe. In that case a Twin Turbo setup would be having two turbos pushing the same amount of boost. You would find this on a Dodge Stealth and Mitsubishi 3000GT. They simply use two equal turbos, in this case TD04s, to push equal amounts of boost. I never understood this setup since using one turbo would be more efficient.
Then there is the Sequential or as you call it the Bi-Turbo setup. They use a smaller turbo that will spool faster at low speed and then they have a larger turbo to pick up when the smaller turbo runs out of speed. This is a very ideal setup. If you are going to do a Twin Turbo system the Sequential is the only way to go. I don't see any practical reasons to go with the standard Twin Turbo setup.