1) Use the canvas size tool and simply double the height, clicking on the middle uppermost of the nine squares to make it expand from the bottom only.
2) After that, simply cut and paste the lower half in.
3) Lower the opacity of the lower half, and zoom in to line up the bit of the two halves which overlap from the scans. You might have to zoom right in, because Potatoshop has a really annoying function where it tries to line it up automatically and does a suppagetti bad job of it.
4) Delete the rubbish-looking top 'lip' of the bottom half, which has probaly gone all wonky and grey from being cut off in the scan.
4) Then just return the opacity back to normal and layer -> flatten image. :p
Sounds complicated but you get used to it. Better than buying a new scanner anyway. Guess I could upload screen shots if you really want, too.