Simple, basic way:
If you don't have a tileset exported:
in RPG Maker, click the import/export button and export the tilesets you want to combine to some folder.
In GIMP,
1. Open one of the tilesets
2. Click Image -> Configure Grid
3. Change height and width to 32 and 32 (I'm assuming this is XP?)
Close out that window
4. Click View -> Show grid
5. Click View -> Snap to grid
6. Click Image -> Canvas Size
7. Change the height to whatever you need, like if you want a whole new tileset, then double it or whatever, but keep it a multiple of 32.
8. Click Layer -> Layer to Image size
9. Open up the other tileset
10. Select all (ctrl + a) and copy (ctrl + c)
11. paste (ctrl + v) it into the extra space in your first tileset.
Now save and import it back into RPG Maker.
Hope that was descriptive enough.