Hello.
Yes, I can see you are having difficulties uploading two at the same time. Since you know how to add an attachment it shouldn´t be too hard. Just as Angell has pointed out, you need to:
1. start a ¨new topic¨
2. click the upload attachment tab below your text box (as you´ve already done)
3. Now you need to browse and then click add the file.
4. Now once you´ve done this you need to repeat step 3 before clicking on ¨submit¨.
Once you do this, your assignment will be graded.
All the best