Hi Batya,
There are a couple of ways you can link multiple test cases to a defect. One way is from the Test Case. If you edit the Work Product field and select the desired defect the Test Cases will all appear on the Test Case tab of the selected Defect.
Alternatively, if you go to the Test Case tab of the Defect you can add the Test Cases from there. If you are using the older Editable Detail Page (EDP) it only supports the creation of a new test case. However, if you do it through the newer Quick Detail Page (QDP) you can choose to add an existing Test Case or create a new one. You can find more information about the QDP here Detail Pages | CA Agile Central Help
I hope it helps!