Apple - 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 & drop the 2 images from Skitch to Keynote
• align the images in Keynote, resize, do some edits if necessary
• take a snapshot with Skitch

Can't do it with Skitch, as noted. But LittleSnapper will work. It creates an image for each screen, then you can drop them all into a photo imaging app and compose into one image.