The USAF did something like it in the 60s by joining up two or more straight wing fighters wing tip to wing tip and it resulted in fuel savings of over 20%. The experiment was called off after a couple of accidents caused by a loss of control by the slave airplane which caused it to roll over onto the lead without decoupling.
Great video of it on the History channel in the US last year.
A good idea for the time, but not really practical for passengers.