Do you mean using one mouthpiece for both bass tuba (tuba in F or Eb) and contrabass tuba (C or Bb)?
If that is what you mean, not many will use the same mouthpiece for both. Most people will use a mouthpiece with a shallower cup for bass tuba. Exceptions include those cases where a player is trying to make a large Eb tuba sound like a contrabass tuba, as is often the case in the UK.
There used to be interview articles with several of the top Los Angeles studio tubists on tubanews.com. Evidently that site no longer exists. I seem to recall that one of them may have used a Marcinkiewicz N-something (maybe an N1 or N2?) on all of their tubas. I don't recall 100%.
Jim Self's website seems to say that he uses one mouthpiece for all of his tubas:
http://www.jimself.com/hardware.htm wrote:
On all my tubas I use a new Yamaha Jim Self signature model made by Hiroaki Imaoka mouthpiece -- which is like a Miraphone C-4 rim with a deeper cup. This mouthpiece was originally designed by Norm Pearson.
I've always used a Bach 24AW on both Eb and BBb tubas. Currently playing around with a Miraphone TU21 to get a more F tuba sound from my rotary Eb but keep going back to my old and faithful 24AW.