Skylab Salyut ? Salyut 6/7 ? Salyut 7/Mir ?
During the Skylab flights the Soviets lost three space stations to launch failures and on-orbit lost of control (DOS-2/DOS-3(Kosmos-557)/Salyut-2), so they only made two flights during May 1973 - February 1974, of which the flight of Soyuz 13 coincides with the stay of Skylab 4.
The last crew on Salyut 6 (Kovalyonok/Savinykh) left on May 26, 1981, and Salyut 7 wasn't launched until April 1982.
The Soyuz T-15 crew (Kizim/Solovyov) went back and forth between Salyut 7 and Mir during their flight in 1986, but the two were never manned simultaneously.