For the sake of argument, I'll assume that there are entries with keys Ker:2005 and Fow:1999 in your .bib file which point to the pieces by Kerievsky and Fowler, respectively.
Assuming you use the natbib citation management package, you could proceed as follows.
Load the natbib package with the options square (to produce square brackets around the parenthetical citations) and numbers (to produce numeric-style citations).
Use a combination of \citep and \citealp to create the desired citation:
Refactoring is "..." \citep[][cited by \citealp{Ker:2005}]{Fow:1999}.
If this syntax looks like it's going to be a bit hard to remember, you might want to define a macro named \citeprimsec in the preamble:
\newcommand{\citeprimsec}[2]{\citep[][cited by \citealp{#2}]{#1}}
and employ it in the body of the document as
Refactoring is "..." \citeprimsec{Fow:1999}{Ker:2005}.
The name of the macro will hopefully make it more or less obvious that the first argument should be the key of the primary reference and that its second argument should be the key of the secondary reference.
Addendum: I just noticed that your comment that you do not use natbib. Well, adding natbib to the list of packages you load in the preamble shouldn't be a big deal. Just be sure to load it with the options square and numbers. You needn't modify any of your existing \cite commands.
Once you compile your document you will need to also compile the bibliography. In TexShop there is a drop-down menu that defaults to LaTeX change it to BibTeX and press typeset (I usually do this twice). Then recompile your document with LaTeX twice and your reference should appear or errors on the BibTeX step.
Best Answer
Does BibDesk have the 'unpublished' option? Will this work in the appropriate fields?
This should work manually in a bib file (with appropriate syntax, of course), so I can only assume it will also in Bibdesk.