Cimbasso is not a double for B-flat/C/E-flat/F tuba, and neither are those different lengths of tubas considered to be doubles.
Alto flute and piccolo are not doubles for flute. They are all flutes.
Clarinets of different lengths are not doubles.
Soprano, alto, tenor, and baritone saxophones are not doubles.
Piccolo trumpet, E flat trumpet, C trumpet, and flugelhorn are not doubles for trumpet. Rather, they are trumpets.
When a second oboe has a movement or two in a symphony or a complete one movement piece where the player moves to English horn, that's not considered a double.
Different fatnesses of B-flat trombones or even E-flat alto trombone are not doubles.
Doubles are typically when a wind player also plays a stringed instrument, keyboard, or percussion - or some other crossover from someone's main instrument to any of these other totally different instrument families.
Crossing over the primary families of woodwinds or brass can be considered doubles, such as people who play Broadway shows that tour and can play the various flutes, the various clarinets, the various saxophones, oboe, and bassoon, because of the completely different ways in how some of them are blown and the fingering systems vary considerably between those five different families of woodwinds.
I suppose it could be argued that playing valved brass instruments and also trombone is a double, because of the slide skills (and the fact that trombone players need to learn how to fake playing legato) vs. the valve skills.
Bass trombone is a trombone, and it's just a big fat B-flat trombone that doesn't play high as easily/clearly and plays low more easily/clearly (just like a big fat tuba versus a skinny tuba), and even its primary auxiliary valve is the same as is found on tenor trombones.
Talking about how bass trombone written parts' musical rolls differ from tenor trombone parts is just part of generally being a well-rounded musician. When saxophonists play their baritone saxophones (or when oboe players pull out their English horns), they don't pontificate about how the parts are different and how they have to wear different musical hats, or how they have to adjust physically, or anything like that. They just do it.
This isn't difficult to understand.
James Markey WAS a tenor player (NY Phil) and SWITCHED to bass (Boston). To do this, there has to be an identifiable difference in hardware, technique, range, attitude, embouchure, everything. It's considered a different chair in an orchestra.
...yet he didn't exit professional life to go back to school and learn how to play completely different instrument, because it's not a completely different instrument, it's simply overwhelmingly selected (being a fatter version of the same instrument) the play the lower third trombone parts. Second violinists didn't have to study to play second violin. and second and third clarinetists didn't have to study to be able to play second and third parts on their instruments either, and bass clarinetists - likely already accomplished clarinetists - MAY HAVE taken a handful of lessons from someone who is known to play the bass clarinet particularly well (tone production, mouthpiece recommendations, excerpts interpretations), but I'm sure they didn't go back to school to study the bass clarinet.
I know just a little bit about "bass" trombones.
- I put one together for myself from what appeared to be a hopelessly destroyed Yamaha 322 bell section and a 60-year-old slide-tuning dual-bore Olds (.554" - .564") slide.
- I've got two bass trombones in here right now (from a university's studio professor) to take their playing slides completely apart, straighten their tubes, and put them back together (hopefully as well or better than they were put together when new), as well as having been asked to "split" their linkages to the in-vogue style of thumb and middle finger separated operation of their valves.
I also know that - when I have to play slide trombone, as I did on this recording session...
https://youtu.be/DKPDt_Ve06g?si=ajz3TxnppOIlfwXq
... that I have to practice my butt off, because accurate slide technique is totally foreign to a valved instrument brass player, whereby slide trombone (IN GENERAL) is - again - a legitimate "DOUBLE" when contrasted with tuba or baritone horn.
(Just for what It's worth, I do NOT consider myself to have mastered the trombone slide well enough to consider myself a doubler on trombone, even though I occasionally find myself playing it.)