I understand it's relatively common to have two instruments, even from different families, share a part and/or space in the score if they play the same thing throughout the whole piece. In a piece I'm transcribing, the trumpet and bass clarinet play the same rhythm at all times, and due to other structural reasons I'm motivated to fit them onto the same staff in the score even though their notes are different. The musicians' parts would still be separate.
The sticking point is that the bass clarinet transposes down an octave. Will the voicing intent be clear if I write it transposed the way it usually looks on a bass clarinet staff? Should I do it differently, or do I need to explicitly indicate one way or the other somehow? (An optional-octave clef maybe?)
Similarly, the staff name: Under different circumstances I would just put the higher instrument name above the lower one and expect that to disambiguate which line is which, since they never cross over each other. But if the clarinet is written above but voices below the trumpet, I'm not sure which ordering is expected or how to make it clear.
Finally, is this a stupid idea? I couldn't find examples or any commentary on whether it's standard practice. I think it's justified in this piece but I'll reconsider if feedback says it's inevitably confusing.