This is not NASAs first time dealing with this type of scenario. The crew of Skylab 3 had thruster issues in their Apollo command module. NASA actually redesigned an Apollo capsule to seat 5 in a return to earth. It went so far as the rescue crew starting to seriously train for a launch. In the end they found workarounds for the issue and brought them home normally.
The rescue kit built for Apollo during Skylab, while a precedent, is not a complete one. Apollo was the only vehicle available in that situation, so if the CSM already at Skylab couldn't be used, the rescue CSM had to launch. There are alternatives to (say) squeezing in more than four people into Crew Dragon.
In case of a true emergency, would squeezing two people into one seat be that dangerous? (As in, is the safety envelope of the vehicle tied to weight in each seat?)
http://www.astronautix.com/s/skylabrescue.html