lord of the rings - How had Saruman "cheated" Sauron?

He's cheated Sauron in the sense that he's pretended to be looking for the Ring in order to give it to Sauron; in fact, as he lets Gandalf know quite early on, his plan is to keep it for himself:

'"And why not, Gandalf?" he whispered. "Why not? The Ruling Ring? If we could command that, then the Power would pass to us. That is in truth why I brought you here. For I have many eyes in my service, and I believe that you know where this precious thing now lies. Is it not so? Or why do the Nine ask for the Shire, and what is your business there?" As he said this a lust which he could not conceal shone suddenly in his eyes.

'"Saruman," I said, standing away from him, "only one hand at a time can wield the One, and you know that well, so do not trouble to say we! But I would not give it, nay, I would not give even news of it to you, now that I learn your mind. You were head of the Council, but you have unmasked yourself at last.'

(Fellowship of the Ring, Book II, Chapter 2, "The Council of Elrond"; emphasis added)

Saruman plans to find the Ring and wield it for himself, taking the power that Sauron put into it. Whether this plan would work is debatable at best; but Saruman might easily have deluded himself into thinking that it would work—even thought of the Ring seems to have allowed it to exert power over the thinker.

There's no particular reason to assume that Saruman is of a lower "sub-order" of Maiar than Sauron, at least within the strict bounds of what was developed at the time of The Lord of the Rings. Certainly the Istari were restricted in their power within the bounds of Arda, but I don't know whether this was a hard enforcement or not (whether they physically could not exert their full power, or were just told not to).

As far as whether the Orcs would tell Sauron of Saruman's taking the Ring, look at how the Orc of Mordor reacted to Sam who was merely carrying the Ring:

His will was too weak and slow to restrain his hand. It dragged at the chain and clutched the Ring. But Sam did not put it on; for even as he clasped it to his breast, an orc came clattering down. Leaping out of a dark opening at the right, it ran towards him. It was no more than six paces from him when, lifting its head, it saw him; and Sam could hear its gasping breath and see the glare in its bloodshot eyes. It stopped short aghast. For what it saw was not a small frightened hobbit trying to hold a steady sword: it saw a great silent shape, cloaked in a grey shadow, looming against the wavering light behind; in one hand it held a sword, the very light of which was a bitter pain, the other was clutched at its breast, but held concealed some nameless menace of power and doom.

(Return of the King, Book VI, Chapter 1, "The Tower of Cirith Ungol")

Saruman is no doubt aware that something of this sort would occur; and given the orcs' reaction to Sam, it's certainly reasonable for Saruman to believe that he could overmaster the Orcs' wills.