I followed these steps to merge Quiz A into Quiz B:

1. Go to course settings > export content
2. Export type: quiz
3. Select the "Quiz A" and download the zip file
4. Go to settings > import content
5. Content type: qti zip file and upload the zip file from step 3
6. Default question bank: create new question bank
7. Edit Quiz B
8. Go to the bottom of the question tab, click the "find questions" button and select the newly imported question bank
9. Select all questions, add them to a new question group in Quiz B, and save