I’d asked this several years ago and at the time it wasn’t possible to bracket two staves without having barlines between them, unless one tricked Dorico by calling them vocal staves and surreptitiously changing the names and samples. Is that still so?
It’s no longer the case. In the left panel of Engrave mode you’ll find tools for brackets, braces and barline joins. They can all work independently. Select something on each stave you want bracketed or barred together, then press the appropriate button in the left panel. Brackets and braces can then be dragged; I don’t think barline joins can be.
Hmm, it doesn’t seem to work that way. I know about the left panel of Engrave mode but choosing a bracket will always cause barlines between the staves (except, of course, for vocal staves). What is new to me is that the barline joins can be removed. Selecting a barline produces handles at the top and bottom. Selecting one of these and pressing Alt and an arrow will remove them between the staves. Nice!
Ok. If we’re going to be pedantic then no, it’s not possible to bracket two staves together without there also being a barline join added. You can then remove that barline join by selecting something on one of the two staves and hitting the Barline Join button.
Sorry Leo, I wasn’t trying to be pedantic. I didn’t realise that barlines between staves could be removed with Alt + up or down arrow. The last time I tried this it wasn’t possible.
I can imagine this thread as coming across as weird. I discovered the ability to remove the barlines only after posting the question, as a search revealed nothing positive.
I do that all the time (search, find nothing, post the question, find an answer / figure it out)
This is a well known technique in software development. You buy a small plastic toy, e.g. a duck, and keep it in your desk. When you get stuck on something, you explain the problem to the duck, and while explaining it you discover the answer.
Internet forums work almost as well, except other people get to see you talking to yourself 
Yes, can be embarrassing sometimes! 
(for reference, documentation links are here - this was added fairly recently to be fair!)
General info about custom brackets/braces/barlines - “custom staff grouping”
Adding custom brackets/braces and adding custom barline joins
Lengthening/shortening custom brackets/braces and ditto for barlines (using the handles)