Currently there is no native way to compare the full content of two commits from within SourceTree, you can only compare single files: If you select multiple files and hit the External Diff button, SourceTree will open a single instances of your diff tool for every file.

While diffing two full commits is a common feature in git (difftool --dir-diff) and can easily implemented via a Custom Action in SourceTree, it would be cool to have this as a native feature of the UI.