I play the trombone for jazz band in the morning and the tuba in the afternoon for marching/concert band. Is this a bad thing to do or is it ok?

in not way is that bad.... heres a thought for you. Tuba increased maximum airflow, not to mention airflow. Trombone smaller embochure that creates more center. both things benefit eachother. Just remember that the bone is just a brass instrument so if you use that same airflow from the tuba on the bone you will have a monster sound.

Yeah, no problem at all. All trombone players should expirement a little on tuba, it teahers you about air, and accuracy.

The only time doubling gets hard is when you go from reed instruments to brass. That gets hard, and is very bad for your embouchure.

hey dude, i play tuba in the morning for marching band and then t-bone later in the day for jazz. i originally started on tuba but thanks to t-bone playing, i have a way-high range on the tuba. both horns (and i think trumpert) use the same muscles. it`s all good.