[samrohan]

"Saruman is a treacherous worm, but the eye is on him"-Grishnakh "

This does not mean that Sauron has any control over Saruman but that he does not trust him and is keeping some surveyance over him.

Also, I never saw the two towers to ever be in some sort of alliance. I have always seen the two troups of Orcs to use each other as a means of backup whilst travelling through Rohan as they are afraid of the horsemen. Afraid being the wrong word as they are not scared but aware of their forces.

Sauron clearly always has the impression he will win over his opponents. Saruman is not a threat but the forces of Gondor is a threat as he believes they have the One ring.