Following the advice provide in the link that Tim gave you will get you almost there. You would need to add two questions randomly from one question bank, then two from another and so forth. Then you also need to decide if you want all of these 20 questions randomly presented. You do this when creating the quiz and choosing between the two options shown in my graphic. This choice could go either way... sometimes one does and sometimes one doesn't. So make sure to think about it. For example, I have created a quiz covering four chapters. I want to randomly pick questions from each chapter, yet I also want to present the questions in chapter order. So for this, I pick "as shown on the edit screen."
Please appreciate all the great programming that went into making these features possible (not by me, but by folks like Tim and others). You can't do these things in Blackboard, for example.
