The procedure is the same, only your starting point is different.
On both the assessment activities you will see these three tabs below the title. Click on RESTRICTIONS.
You will see three sets of dates: due date, start date, and end date.
For granting special access you will want to change the due date and end date from what you originally indicated. DO NOT SAVE THE ACTIVITY YET.
Skip down to Special Access and make sure to tick the appropriate circle. Tick the first one if there is a need for submission and tick the second if you only want your student to see the activity.
Click on Add Users to Special Access button. This will display the list of your students for this course. Select the students you want to grant special access to by ticking on the checkbox preceding their name.
The students you selected will now appear below the Add Users to Special Access button.
Save and close.
Go to the Quiz or Dropbox and select Restrictions. Choose your Special Access settings and click Add Users to Special Access.
Enter the Due Date and Availability, then select the Users to be granted Special Access.
Click Save to finish.