Bykovsky and Jahn
Lev Dyomin and Gennadi Sarafanov blasted off in Soyuz 15 August 26, 1974 on their way to the Salyut 3. When they arrived at the space station, the electronic docking system malfunctioned and the crew had to return to Earth, because they didn't have enough fuel to continue docking attempts. The Soviet news agency TASS stated the cosmonauts were merely 'practicing docking techniques'. They landed August 28.
On August 26, 1978, Valery Bykovsky and Sigmund Jahn (the first German cosmonaut) took off in Soyuz 31 to Salyut 6 space station with more success than Soyuz 15. They swapped out the crew already on the station. They returned November 2.