Thanks for an easy question! Does UT overlap with original Toolbox? Here and there, yes. For example, in Toolbox we name about 20 cities and 20 lands. In UT we name nearly a hundred of both and none of those entries are taken from original Toolbox.
That's one example. Here's another.
In the first book we had one table called "crime and punishment". In Ultimate Toolbox we have prisoners, crimes 1 & 2, local criminal gangs, punishment 1 & 2, an essay on running fantasy criminal trials, types of trials, trial complications, sentencing, arbiters, simple and complex bribes, etc.
We cover topics from the first book in heavier detail in the second, with very, very little duplication at all.
Hope this helps: Having both books isn't necessary. Today, given the choice, Ultimate Toolbox is the bigger, more detailed of the two, and you could tackle all topics with it. If you happen to have the first as well, then you simply have that many more charts and ideas to pool from!
-DM Jeff