I guess that depends on how hardcore you want to be and how much torture you can take. Generally the only 12ers that are linked are Church and Donaldson. The 7 LRR 12ers have been linked before but with a LOT of forethought and preparation. There is some REALLY gnarly territory on the ridges between the peaks, a lot of class 4 and 5 stuff. It can be done but it is very hard.
http://idahosummits.com/epics/dave_rob_12ers.htm