To merge 2 quizzes together, go to the Quizzes section, then check the box next to the two quizzes, then select Merge.
A box will pop up, where you can name the new quiz and select which questions you'd like to come first. Then hit Merge.
A new merged quiz will be created in your quiz list. The two quizzes you merged will still be in your list as separate quizzes too. You can decide if you'd like to keep these quizzes or Delete them.