Not quite as useful as it initially appears.
This will offer some very interesting possibilities for creating an interactive network of brains
When I first found out about having multiple tabs and multiple brains open at the same time in TB9, I thought that this feature would solve the dilemma of having one mega-brain vs. multiple specialized brains. The second choice is obviously better, right? I can create several interconnected brains, each of which has its own set of Types, Tags and Link Types. Much better than using very long lists in a megabrain that contains many different types of information. Easier to search, easier to maintain, and more powerful.
There are several problems:
1. A link to another brain is not the same as a link between thoughts in the same brain. There's no obvious indication that a thought is linked to another thought. There's a URL link that shows up in Attachments and it has a Brain icon but the link doesn't show the name of the other brain.
2. When you activate a Thought that has a link to another brain, you can't see any of the thoughts in the other brain that are linked to the active Thought. You have to click on the relevant URL link and go to the other brain, at which point you can't see the connected thoughts in the first brain. (You can try playing around with multiple windows to see two or more brains at the same time but this is cumbersome.)
3. Links are just one way. If you link "Thought 1" in "Brain A" to "Thought 1" in "Brain B", there's no automatic link in Brain B that takes you back to Brain A. You have to create a second link.
4. There will probably be some overlap among the various brains in terms of Thought hierarchies, Types, Tags and Link Types (unless each brain is highly specialized) and it becomes confusing to remember which brain has what.
5. It is difficult (thought not impossible) to use the "comma trick" when linking multiple brains.
5. You can't search multiple brains at the same time. You have to remember which brain has a particular thought, or run multiple searches.
6. You can't look for "Duplicate Names" across multiple brains, or create any Reports that access multiple brains.
7. You can't see the "big picture" of all your thoughts in Outline and Mind Map view, unless you do a ton of cross-linking.
Because of the above limitations, I've gone back to one main brain and one other highly specialized brain.