While I dont have the book with me this is what i think (Keep in mind it has been quite some time that I discussed this or read the book so i may be wrong). Saruman did indeed send a small army to Edoras (which makes sense since according to his original strategy that is where the King Theoden would be present, He had not yet known that Gandalf had arrived or that he had cured Thoeden) and it was Eowyn that defended the halls in Theodens place and earned the title of shieldmaiden. After all in 10,000 riders they would only keep only 1000 in Edoras their capital?
Also Sauron and Saruman were not allies. They only joined hands a couple of times when their interests coincided but in they were enemies each trying to usurp the other and gain absolute power Alone.