This isn't really a theory. Many people believe that Taim is Demandred, or that Dashiva is Os'angar. Just one thing from the beginning of aCOS. Taim begs Rand to take along some more Asha'man, and Rand chooses Dashiva out of the blue. However, Taim does not want Rand to take Dashiva, and he comes up with several excuses. He also was surprised at first, and Perrin says that he smelled 'surprised and irritated.' Rand became more confident about his choice when he saw that Dashiva was opposed to it. I believe that Taim wanted Asha'man who were under his control to guard Rand, and Rand may have chosen the newer ones because they woud be more likely to follow his orders than Taim's. Taim already has a few who he would like Rand to take chosen out. This could support the Taim-Demandred theory, yet it could also mean that Taim just wants to know what Rand is doing at all times for his personal profit. Who knows?