How can I combine multiple screenshots in Skitch?
Nope, not possible within Skitch. 

The quickest workaround I've found to achieve that is using Keynote as a canvas, so you can combine the image in no time : 


open a new 'White' theme in Keynote
drag &amp; drop the 2 images from Skitch to Keynote
align the images in Keynote, resize, do some edits if necessary
take a snapshot with Skitch