Open SourceTree (v3.0.8)

Use "Beyond Compare" (v.4.x) as external diff viewer.

Select multiple (2) modified files and call external diff (CTRL+D)
Two tabs are opened in BC4 but only the first one shows the differences. In the second tab the remote file is missing.

Another Issue:

Configure a userdefined diff-viewer (e.g.  BComp,exe).
Close Sourcetree and check common git settings for difftool.
Restart Sourcetree
     --> The difftool command in ".gitconfig" will be erased!!!
Open the Options and close it with OK.
    --> The difftool command in ".gitconfig" will be restored!!