I don't own a Mercedes or BMW, but during a brief period when I had two cars, each with half full gas tanks, I've actually done this. Without help. At 2:00 in the morning.The irony. I was getting gas at a Kroger's (grocery store,offering a dime off gas for each hundred spent on food) when a brand new high end Mercedes and BMW pull up on opposite sides of a pump. I watch the dbag fill both cars on one transaction to save the 1.50 to 3.00.
Drove first car to store. Parked it out back. Walked home (half a mile). Got second car and drove up to the pumps. Got first car and parked it so I could hit both gas tanks with the same hose. Filled both tanks. Reparked the first car out back. Drove the second car home. Walked back to the store. Drove the first car home. Whole process took about 40 minutes.