Loughner probably can't be tried because he's severely schizophrenic.
Holmes, seems like he was a lot more meticulous.
Hey, another rule we need to get rid of... letting these people refuse medication to control their symptoms.
Lougher won't be tried because he is incompetent to stand trial. Holmes is incompetent to stand trial. Even if, and if you want to use a hypothetical, even if both were sane enough the commit these acts at the time they did them, they are now too insane to participate in a trial.
I suspect both of them will be put on trial because the standard for insanity is ridiculously high.
It's been over a year and Loughner still hasn't come to trial and it is acknowledged that he may never come to trial. Holmes likely will never come to trial. The standard to claim an insanity defense has nothing to do with competency to stand trial.