Sub-brackets on system, for similar instruments (or nonsimilar instruments)?

I received a score suggestion that I could add sub-bracket ‘around’ my Trombone and Bass Trombone. I couldn’t figure out how to do this, perhaps it is not possible. I ended up creating Player Groups (I believe) which added text & unconnected brackets to the far left, but that was not really what was desired. Is there a way to place the sub-brackets on the Trombones (like the Trumpet 1 and Trumpet 2 already have) ?

Perhaps the sub-bracket around these doesn’t work because the trombones are not considered ‘the same instrument’? So maybe if I ‘deleted’ the Bass Trombone and re-made that part as a Trombone, so it is viewed as the same instrument, and then mucked with the VST and labels to change the appearance back into a Bass Trombone, it would sub-bracket these “same” instruments?

Manual’s example which shows sub-brackets around both instrument pairs, Violin 1/2 and around Cello 1/2:

One can add brackets and sub-brackets in Engrave Mode. Click them onto one staff and then stretch them to cover the rest.

Trombone and bass trombone are not the same instrument type, so Dorico won’t add a sub-bracket around them by default. On a separate note, I think perhaps the group labels are redundant with this kind of ensemble: they’re just using up valuable horizontal space that would be better used for the music itself.