I have an xml + xslt 2.0 transformed into fo using Saxon 8 and want to use
FOP 0.92b to transform into pdf.

Can I do this in a single step (via windows command line), or do I have to
save the xml + xslt to a file and then use this as the source for the fo >
pdf?

You can do this in a single step. You can specify the XML source and an
XSLT on the command line, see
 http://xmlgraphics.apache.org/fop/0.92/running.html#standalone-start

A simple way to use Saxon by default as XSLT processor is to drop
the Saxon jar into the lib/endorsed directory in your JRE installation
(provided you use SUN's JRE 1.4 or later).