The tip and diagrams below are from: https://www.petzl.com/US/en/Sport/Accessing-an-exposed-rappel-station
Note - This post discusses techniques and methods used in vertical rope work. If you do them wrong, you could die. Always practice vertical rope techniques under the supervision of an experienced climber, and ideally in a progression: from flat ground, to staircase, to vertical close to the ground before you ever try them in a real climbing situation.
The amazing and informative Petzl website is full of great Crafty Rope Tricks (CRT). Here’s a good one on accessing an exposed rappel station. It's written for canyoneering, but it could also apply in some rock climbing situations. (There are various ways to accomplish this, this is just one.) Note: canyoneers like using rope bags, as shown here, but it’s not required.
Step 1 - Set up some sort of an anchor in a safe position away from the cliff edge. (Note that any anchor material you use will be left behind.)
Step 2 - The first person ties into the end of the rope, and then lowers themselves through their rappel device to the rappel anchor on the cliff edge.
Step 3 - After securing themselves with a leash/lanyard to the anchor, the first person ties the end of the rope and a loop of the rope into the rappel anchor, fixing the two strands.
Step 4 - The second person can now safely descend to the rappel anchor, either with two leashes and carabiners, as shown here, or down climbing with friction knots (or maybe a combination.)
Step 5: Once both climbers are secured to the rappel anchor, the rope can be untied and pulled from the upper anchor, and then set for the actual rappel.