How can I combine two non-sequential git commits into one?

First do git rebase -i aaaaaa^ and then your text editor will show up looking like this:

pick aaaaaa
pick bbbbbb
pick cccccc
pick dddddd

Change it so that it looks like

pick aaaaaa
squash cccccc
pick bbbbbb
pick dddddd

and close it and git does the rest.

http://git-scm.com/docs/git-rebase