I have made a "How to Combine Tilesets" tutorial.
This tutorial teaches you how to create a larger tileset full of more options to choose from by taking two or more tilesets and editing them to be compatible with RPG Maker XP's tile selection tool.
Its about 21 step by step pages full of pictures to help you see where to go to find certaintools in the RPG Maker, what to do, and what some of your images should look like throughout the tutorial. It also includes premade files for certain things created (from the tutorial) just incase you get stuck or lost.
Winrar - (Word Document and Tileset Files)
http://www.megaupload.com/?d=LORB11LLLet me know what I should fix and/or edit
Hope this helps!
UPDATE: The file now contains the Tileset Template