For bass trombone, I'm new with two triggers, and I was confused about the positions between low E and pedal Bb, and the notes below pedal Bb. Are they the same as with one trigger, or are there alternative positions?
The first trigger is pretty standard, it adds in enough tubing to lower the pitch by a perfect 4th, the equivalent of extending the slide from 1st to 6th position. You get a trombone 'in B♭/F'.
The second trigger is less standard. It may be additive to the first, adding a relatively short extra length of tubing. So you get an instrument 'in B♭/F/E♭'. Or it may be independent, typically adding rather less extra tubing than the F trigger, enough to put the instrument 'in G♭'. Overall you get an instrument 'in B♭/F/G♭/D'.
The 'additive' second trigger gives a lighter instrument. The 'independent' one allows some extra low notes.
Look at the choice of double--trigger bass trombone here: https://www.musicarts.com/double-trigger-bass-trombones
But don't get frightened! It doesn't give you three new instruments to learn! You still read the same music, basically play the same positions. It's just that in the extreme low range you get some additional options regarding where to put the slide/use the triggers when filling in the range down to pedal B♭, and maybe the choice of playing a few notes down from B♭ as 'normal' rather than 'pedal' notes.
To answer your direct question, no, positions for low E down to pedal B♭ won't be the same as with just one trigger. Just as you learned some 'long' positions for use with the single trigger, you'll be learning some even 'longer' ones for use with both. But don't worry. There's only a few notes down there. Each one (or most of them) can be played with one trigger and the slide THERE, or both and the slide THERE. Learn where THERE is for each instance. It's not hard.
(And it's like every note on the trombone, really. The right place for the slide is where THAT note is in tune. Not necessarily where you put a chalk mark labelled '5th position' :-)
If you have just one trigger it takes the trombone down a fourth to F.
If you have two triggers the first one does exactly the same (down a fourth) so you still have all those positions. Plus you have all the positions the second trigger adds.
You'd play the notes below low B-flat down to G-flat with no trigger, so those positions stay the same.
Sort of. With the slide in 1st position, the single trigger takes you down a perfect 4th. With the slide in 4th position, it takes you down rather less, which is why we have to use 'long' positions with the trigger.– LaurenceNov 4, 2022 at 21:59