However, it is much easier in PhotoImpact if you can get hold of that!
In PSP the basic procedure is to create a new, blank image, making sure it's large enough. Open each of the images you want to join, then paste the 2 "halves" into the new image. You have to use the layer palette controls to get the adjustments correct.
It's not someting I've ever done "in anger" in PSP... I did do it when I first got it, following the tutorial in the book... but as I say it's much easier in PhotoImpact, (which I got free on a coverdisc some time ago), so this function I usually perform in that.
I hope this helps