I would expect Sunday January 2nd to be quite busy, as that is the last day of the holiday season and, if I recall correctly, on the last day of the season this past January, there was a surge of activity. I think that there are many people (possibly locals, possibly not) who will go on 1/2/11 for one last holiday experience before all the decorations come down, not to mention that there will be people who arrived for New Year's who are still there, finishing out the weekend.
Definitely make a point of seeing Haunted Mansion Holiday, Santa's Reindeer Round-Up and It's a Small World Holiday on January 2nd. Don't wait until the 3rd, because at least 2 of those things - if not all 3 - will come down immediately when the season ends the next day. What happened in January of this year was that IASW Holiday stayed up until mid-to-late January (after MLK Day), while the Haunted Mansion Holiday overlay was uninstalled. I guess that DLR did not want both rides to be down at the same time, so they chose to leave up IASWH.
But in 2011, we have no clue what will happen. It seems that every time we think we have DLR pegged to certain patterns, they switch things up on us! In January 2011, they may not leave IASWH up past the season ending date at all. They may shut it down when they shut down HMH. There is no telling. So try to get to both of those on 1/2/11 just to be on the safe side.
You can actually get DCA done in less than a full day, so keep the option of Park Hopping between both parks on both days open. You may need to squeeze in some extra things in DL on your dedicated DCA day. Likewise, sometimes DCA can be a welcome escape from the madness of DL.