They're limited to replacing entries in two disciplines between the team event SP/SD and FS/FD.
From the Qualification Procedures document:
In Sochi, Russia chose to use two Pairs (Volosozhar/Trankov and Stolbova/Klimov) and two Ice Dance (Bobrova/Soloviev and Ilinykh/Katsalapov). They could have optioned for Sotnikova to do a segment, but they'd have had to have Pairs or Dance do both segments.
So only 2 disciplines are allowed to use 2 athletes/pairs. Strange.