Well, I think he knew that a halfling had the ring because that is what Saruman's and Sauron's orcs were sent out to find.
But, I really don't think he ever had a clue that two halflings were on his back doorstep with the ring in tow, because he could've easily caught them.
I think he was diverted by the capture of Merry and Pippin and believed them to have the ring. This was probably even more true after Frodo was captured and they found no ring on him at all - and after Pippin looked in the Palantir. Besides, they didn't know Frodo was accompanied by another halfling... I believe they thought they were looking for a fierce Elven warrior (Sam loved that!).
Sauron probably didn't know what hit him!
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