Lately I've been speculating in this and other posts about how a U.S. shadow cabinet might work.
One complication, as I see it, is that it would be difficult or impossible for presidential candidates to carry on the common practice of naming at least one member of the opposition party to the Cabinet.
It seems to me that it would be much more difficult for a potential Cabinet member to accept an appointment from the other party's nominee in August of the election year, than it is to take a job from a president-elect of the other party after the election.
In a rare case, such as that of Senator Joe Lieberman, an "Independent Democrat" who had already come out in support of Republican nominee John McCain before the party conventions, a politician from the other party might feel free to accept a shadow cabinet role. But, for the most part, that would be seen as an act of disloyalty, and most politicians would probably shrink from it.
Also, some bipartisan tokens in recent times were already a Cabinet member under a president of one party, before being appointed by a president-elect of the other party. Norman Mineta was secretary of commerce during the last few months of Bill Clinton's presidency, and was subsequently named secretary of transportation, as the only Democratic member of George W. Bush's Cabinet. And Robert Gates, who has been secretary of defense during Bush's last two years, is Barack Obama's choice to continue in that job in his administration.
One could certainly not simultaneously be a member of a Cabinet of one party, and a shadow cabinet of the other party. If a Cabinet member disagreed with administration policy strongly enough, he or she could resign, and then go over to the other party. Such was not the case with Mineta and Gates. However, Colin Powell, who was secretary of state during Bush's first term, endorsed Obama before the election, apparently though, not with the intent on either side that he would take a job in the Obama Administration.
I've previously written that I'm not a big fan of the bipartisan Cabinet idea. But, anyone advocating implementation of a shadow cabinet system in the U.S. would need to take into account the difficulty of reconciling that with the bipartisan concept.