However, there are also disadvatages to having different use years and separate memberships. For instance, points from the two contracts could not be combined for any one night without using your annual transfer allotment. Also, as you can only transfer 2007 points to 2007 and 2008 to 2008, etc., there will be a limited window in which to do this transfer.
For many people, the cons outweigh the pros, unless you are certain you will travel at the same time annually, and to a specific resort. But there are also those that manage well. For the average member, I think it probably isn't a good idea to have separate contracts and use years.
If you buy a resale with the same use year, and title the same as your orignal contract, Disney can combine them under the same membership number, be sure to have the closing agent request that, and it never hurts to talk to member accounting BEFORE closing.