a simple way is to use two image files and a mask.  You may be able to omit the mask.

add a blender node. connect the two image maps to inputs 1 and 2 . set the blend value (last input) to 0.5 and connect the blender node to diffuse.

The blend value can be adjusted as needed.

For better blend control make a black and white mask and plug it into the blend value input.  The mask sets one or the other image to display. Blur the edge of the mask a little and you get a transition between the two textures.