You are provided with an opportunity to access ExamOnline, complete your technical setup and make sure you are ready for your actual exam day.
You only need to complete one mock exam, prior to your actual exam.
Were you able to take a photograph of yourself to authorise yourself? Were you able to complete the mock exam?
If it's a yes to both, you are good to go!
If you feel you need more practice using the platform, then you are welcome to go through the second one, if available. But it is not necessary at all. Once you 'Submit' an exam, you will not be permitted to access that exam ever again.