This is a timely question, actually. Currently, you can only report on one fact table at a time. This restriction was historically due to complexities in reporting, queries, etc. Anyway, over the past 2 years, Ariba has been investing heavily in what is called "Enhanced Multi-Fact Reporting" which will provide you what you seek. This feature is scheduled to be released in our 11s1 release for Contract Management (and all On Demand Products). You should see communications arriving soon about Early Access for the 11s1 release, as well as firmer dates for the release, but it is scheduled currently for the first half of 2011.
Hope this helps!