A single AOS will do for both of them. No need for two AOS. Has your brother been selected for further processing? If yes, did he include his wife in the forms that he sent back to KCC? Or is he planning to include her for the interview?
As has been elaborated a lot on this forum, the COs first presume an intent of marriage for the purposes on immigrating in such matters. If the marriage is real, then your brother should not be worried at all. If not, these people have a way of extracting inconsistencies and using that as a basis to deny visa issuance. In all these cases it will be probably safe for him to do it alone and come for his wife later on. A little bit of elaboration on the case will afford us the chance to advice appropriately.
@Bennit, I hope preparations are ongoing very smoothly. The lord is on your side and you will be successful in you interview. Stay blessed.
Thanks, bro. Yes, preparation is seriously underway. I'm trying my maximum best and will leave the rest to God. Please, remember me in your prayers. Peace unto u! Benniit