Just select 2 commits in the log at once using Cmd-Click, then you'll see the complete diff between those two commits.
If you want to filter things down further you can right-click on a file, click 'Log Selected' to get the history just for that file, and then again multi-select commits in that window.
Follow-up question: Selecting two commit in the log at once work correct, but it occurs often that the selected commit are lost while viewing the diff (it returns to the uncommited changes). Would there be a way to open a new windows with the diff of the selected two commits? Thanks!