Assignment Groups Weighting is used for the big categories of grades which you will use to calculate your students' final grades. For example:
Homework Assignments: 20%
Weekly Quizzes: 10%
Chapter Exams: 30%
Midterm Exam: 15%
Final Exam 25%
These are the categories that you would list on your syllabus.
Now, it's the important part: there is no sub-tier of percentage "weighting" within each category. Now, you need to look at points.
Let's say, for example, you have five chapter exams, and you want them to be counted equally. Then each exam would need the same number of points. (e.g. 30 points each)
However, perhaps, you want the last two chapter exams to be worth more. Then you need to adjust the number of points. (e.g. 30 points, 30 points, 30 points, 50 points, 50 points).
So, if you have two quizzes, and the second one is more important, just give it more points.