Does anyone happen to have an idea for a possible workaround to imitate this “double bracket” which joins the vocal staves in the BGA editions?
No, I’m afraid I can’t think of a good way of achieving this in Dorico directly. You could do it in a graphics program after completing the project, perhaps.
Thank you, Daniel! Maybe if once you implement the feature of changing the design of individual barlines (I’ve just read that it was requested in another thread), that could be a workaround for doing this by changing the design of the systemic barline and just keeping the default bracketing settings.
Hi. I would like to have a long bracketing over Glockenspiel, Marimba and Tubular Bells, indicating they will all be played by the same percussionist. But I would like to keep the original brace for the two-staves Marimba (left image). However, the closer to this I can do is having a sub-bracket (right image). How can I achieve the first option (left image)?
You can have sub-brackets appear as braces instead, although this will apply to all secondary brackets in the layout.
Ok, so not possible to do this just for the Marimba?
It would be better to have it only for instruments that use two staves, instead of putting it to Violins I and II.
On that same page in Layout Options, you can choose whether or not you want to show secondary brackets for adjacent identical instruments 
Well I do, but I would like to have the sub-brackets for the Violins I and II and the brace for the Marimba
It would be nice to have this option in the future.
Would group brackets do a suitable job?
Mark, if you are asking me (which you probably are not) I would like to see the ability to select whether sub-brackets appear as brackets or braces for just the situation Coe describes in the post before mine.