Constructed lists are the way to go here. After you've created your first select question, go into the Lists Manager. There should be a button for it near the top of the main window or Write Questionnaire window. You should see the list for the first select question already in there.
Click to create a new constructed list. You should set its parent list to the list being used by the first select question, then give it these instructions:
AIC(Q1)
"Q1" should be replaced with the name of the first select question. (The "AIC" stands for "add if chosen.")
In your second select question, go into the item settings and set the question to use this constructed list and you should be good to go.