Hello, fellow bass player here. If you're using the kind of horn that I think you are, then it has dual rotors.
The one you use with your thumb is an F trigger. This takes everything you play with the trigger down(or up) 5 positions i.e. 1st position + F trigger = 6th position.
The other trigger(if you have it)is most likely a G flat or D. However, some 2nd triggers are dependent on the 1st trigger. This means that the 2nd won't work unless you use the F at the same time. Try blowing air through your horn and press the 2bd trigger and see if the air flow changes. If not than it is dependent on the other trigger.
If you're new at bass trombone, don't worry so much about the 2nd trigger unless you play below the staff. Also if you want good bass bone music, look up cello or tuba parts. Hope this helps!
<Added>P.S. found a bass chart site:
http://www.yeodoug.com/resources/faq/faq_text/slidechart.html
