Saruman for or against Sauron?

The Two Towers deals with the link between Isengard and Mordor in the battle against ME. However, one thing that I have had trouble deciding was, did Saruman JOIN with Sauron (as with the film), or was there just a casual link between the two (i.e passing of information) from which the treacherous Sauron benefited totally through commanding the will of others from the seeing stones?

I had the idea that Sauron was just using Saruman as a puppet to turn against ME, hence increase his own chances of defeating ME, while never having any plans of formally joining with Saruman. There is no clear evidence that Saruman built his army to directly aid Mordor as far as I have known.