A lot depends upon how you want to blend them. Are you after something like a collage effect? Inserting one picture into another? Without seeing an example of what you're after, hard to offer more definative advise.
Presuming they're on seperate layers, for a collage I'd is the highligter (marquee?) tool to highlight the area I want to get rid of, then delete it (or use the eraser tool), leaving yourself some wiggle room to trim up and fine tune later. Then I'd overlap and move the two layers around to about how you want the final image to look. Then use the soft-edged eraser (or even a soft edged highlighter) to eliminate very gradually the edges.
Inserting one picture in another is easy. Put the inserted picture in a layer above the base picture, then erase (through the various means) what you don't want. One trick is to adjust the opacity of the upper picture so that you can see the lower picture through it to aid in erasing.
here's one I did, just for grins . . .
