[Boromir88]

That's a bold statement to make, Master Dwarf!

Not really, considering the quote provided from Letter 246 where 'only' Gandalf would be expected to master the Ring. And also Sauron knew very well what Saruman was up to, but Sauron being able to use him only as a pawn in his desire to dominate:

'Saruman is a fool, and a dirty treacherous fool. But the Great Eye is on him.'~The Uruk-hai

The big difference between 'good' and 'evil' is the evils inability to 'bond.' 'Good' can ally themselves and come together for a common goal, a common purpose, and a common enemy. Where 'evil' is just all in it for themselves. They may join together, because it would make their goals a lot easier, but behind all the 'buddy-buddy' they are just planning the other's downfall. Because after all there can't be two dark-lords ruling can there be?

I'll take when Sauron joins with Morgoth. A lot of people think that Morgoth sort of sullied or persuaded Sauron to join him. But, actually I think Sauron, much like Saruman was to him, was drawn to Morgoth and really was forming his own plans of ruling. And Sauron being smart knew that the best course for HIMSELF to rule would be to side with Morgoth for the time being...just as Saruman does with Sauron.

'It was the apparent will and power of Melkor to effect his designs quickly and masterfully that had first attracted Sauron to him.'~Morgoth's Ring, Myths Transformed

'Sauron had been attached to the greatest, Melkor, who ultimately became the inevitable Rebel and self-worshipper of mythologies that begin with a transcendent unique Creator.~'Letter 200

I think both of these show that Morgoth did not sort of sully Sauron to his side, but Sauron was attracted to Morgoth's power and will, and he soon attached himself to Morgoth.

Now why would Sauron do this? Well, because I feel like Sauron had his own aspirations of ruling, just as Saruman did. And Sauron knew that Morgoth was a much more inherently powerful being than he was, so in order to achieve his own goal of ruling, he'd join up with Morgoth, and eventually break away, and this is exactly whate he does:

Sauron was not a ’sincere’ atheist, but he preached atheism, because it weakened resistance to himself (and he had ceased to fear God’s action in Arda)..... To wean one of the God-fearing from their allegiance it is best to propound another unseen object of allegiance and another hope of benefits; propound to him a Lord who will sanction what he desires and not forbid it. Sauron, apparently a defeated rival for world-power, now a mere hostage, can hardly propound himself; but as the former servant and disciple of Melkor, the worship of Melkor will raise him from hostage to high priest.~Morgoth’s Ring, Myths Transformed

Sauron did not 'sincerely' believe what Morgoth did or taught. But he knew if he used Morgoth's names and ideas that would make his goal of ruling a lot easier, as people would be drawn to him more. And this is exactly what happens:

'When he found how greatly his knowledge was admired by all other rational creatures and how easy it was to influence them, his pride became boundless. By the end of the Second Age he assumed the position of Morgoth's representative. By the end of the Third Age he claimed to be Morgoth returned.'~Letter 183

As people start listening to him, and resistance was weakening, he was no longer Morgoth's 'servant,' his pride grew and eventually he conceived himself as being Morgoth himself.

Anway, where I'm going with all this is showing the inability for evil guys to bond together. They're in it for themselves. Sauron was drawn in by Morgoth's power, but Sauron himself had his own plans and goals, and he took the best route in joining Morgoth's side, eventhough he necessarily did not agree with his ideas. And this is exactly what Saruman does with Sauron:

A new Power is rising. Against it the old allies and policies will not avail us at all. There is no hope left in Elves or dying Numenor. This then is one choice before you, before us. We may join with that Power. It would be wise, Gandalf. There is hope that way. Its victory is at hand; and there will be rich reward for those that aided it. As the Power grows, its proved friends will also grow, and the Wise, such as you and I, may with patience come at last to direct its courses, to control it. We can bide our time, we can keep our thoughts in are hearts, deploring whatever evils done by the way, but approving the high and ultimate purpse: Knowledge, Rule, Order;....~The Council of Elrond

To make my long story short (sometimes I can go off a bit, let's see if I can tie this up). 'Evil' is all in it for themselves, Saruman planned on backstabbing Sauron, and Sauron knew very well what Saruman wanted to do. Seeing all this, I agree with Kuru in that Sauron had Saruman beaten in every aspect, in power, strength of numbers, and knowing what Saruman was up to, and playing Saruman to suit his own advantages. Going back to what Grishnakh said, Saruman was being fooled.