If you put your terrains on two layers, overlap them a bit, add a blank layer mask to the topmost layer and use a grungy brush set to low-mid opacity to stroke the outer edge of the overlap (on the layer mask) to soften the transition... that should work in the GIMP. It'll work in any painting program that has layers and layer masking capabilities. The other option, which is more unforgivable if you change your mind but works without masking, is to put the terrains on two layers and use a grungy brush eraser on the topmost layer to the same effect. There's probably an uncountable number of ways to achieve a similar effect... the wonder of digital media. This way is a little more hands-on.