I failed one part of the combined exam - do I need to retake both parts?

You only have to retake the failed part.  On receipt of a pass result, we will combine it to the existing pass result for the other part to produce an overall pass for the TOGAF 9 Certified level.  A candidate must wait a period of one (1) month before a retake.

http://www.opengroup.org/certifications/faqs