Dependent fields are not a feature in osTicket at this time.
The only way that I can think of to get around this within the ui would be to use Help Topic to include the custom form that asks the second level question.
Alternatively you would need to: edit the core files; create a plugin that lets this behavior happen; or buy a third party plugin to achieve your goal.
I believe that the paid 3rd party plugin Superlists does what you are asking for, but I have never used it or evaluated it.
See the Resource list I maintain for osTicket: