There are two players: A practices serving from the deuce side, B returning every serve with a lob down the line.
The reason player B returns down the line is because in a real doubles match situation his opposing net player would be inside the ad service box.
To make the drill more challenging, a line of cones should be placed about 6 – 8 feet inside and parallel to the baseline; player B should aim his lobs to land between the baseline and cones.
After the returner gets 7 lobs deep, they can switch to the ad side.
Players should change roles as well: server will take his turn to practice the lobs too.
Note:
The lob is one of the most “hated” shots in doubles therefore… it must be practiced and used especially as a serve return option.