TromboneChat

We are likely to do the next program with only 2 trombones. Mixed program. Would you go principal and 2nd or principal and bass? We are expecting a tuba.

Depends on the group and the individual arrangements. Sometimes a low trumpet part doubles the 1st trombone, so you could make it 2nd and bass

In general I wouldn't skip a part, 1st and bass are too far apart if there's nothing in between.

Will depends on the pieces.
We did our Christmas concert with 5 brasses: 2 horns, 2 trumpets (myself on principal) and 1 trombone. For 2 pieces I switched to my bass trombone to play bass part in Pirates of the Carribeans and 2nd part in a jazzy tune. It was the best mix of "doing with what we have" as discussed with the MD.

Second the opinion that it depends on the pieces. Some old classical pieces are written so the 3rd has more cues for other (possibly) missing instruments. Some "school level" arrangements have only 2 trombones. And it's not uncommon for the horns to double trombone parts.

As for hardware, at least one of you should have an instrument with an F-attachment to cover lower parts. And there may be times when you will play using the tuba to cover 3rd trombone when there is no tuba part.

One pops style concert we played a couple of movements from the Hamilton-Hardy arrangement of "Water Music" using 2 horns on the 1st and 2nd parts and two trombones on 3rd and 4th horn parts. Came out really well.

There are a number of pieces on the program, including Tristan Prelude, H&G overture, Liszt’s Preludes, Lemmerkainen and something by a local composer. So, a mixed bag probably.

This - In general I wouldn't skip a part, 1st and bass are too far apart if there's nothing in between - strikes me as good advice.

I'd go with first and whichever of 2nd and Bass has the best stuff to play.

On the other hand, 1st and bass gives you two different voices rather than one.

Get a score. Decide which phrases and notes are covered by another member of the brass section (especially horns and tuba, but also low trumpet) or woodwind section (especially bassoon or a low clarinet). Eliminate the doubled/unison sections and notes doubled elsewhere. Then re-write the parts for 2 trombones, preferably tenor and bass.

If you go bass the odds are they’ll be able to cover whatever the 2nd part is either way, if what you’re asking is if you should book a tenor bass player.