[Boromir88]

I've often compared the Saruman-Sauron relationship like the Hitler-Stalin relationship.

Hitler and Stalin necessarily didn't like eachother (they both had contrasting views) but they agreed to put their differences aside and not attack eachother. For a long time they would help eachother out, but both had plans of undermining eachother once the allies were gone. Hitler knew the two-front war was one reason for Germany's defeat in WW1 so he signs this pact with Russia trying to prevent that again. In secret he just wants to deal with the West first and then turn on Stalin. It was when Hitler believed that the West had been dealt with (one fault) when he turned on Stalin and started invading Russia (another fault).

I think the Sauron and Saruman relationship can be similar to this. They both are after the same thing (The Ring) and they enter into this agreeance of "Look we both hate these people let's get rid of them," but they were secretly planning to betray eachother if and when they finished with Gondor and Rohan.