As Shawn says, I don't think it's possible to do that with the Snipping Tool.
As a somewhat cumbersome workaround, if I have something that I want to capture that requires screen scrolling, I use Snipping Tool to capture a piece of it, then scroll to see the remainder of the image and take another snip. I don't bother saving the
second snip, but just click "Copy" in Snipping Tool's Edit menu
I open the first snip in IrfanView (http://www.irfanview.com/) and then use IrfanView's "Paste special" feature to paste the second snip at the bottom, left, or right side of the first
snip as appropriate.
It actually is easier to do than to describe.
