Aaah, right. She still hasn't told Sayaka about their alliance.

"I know this may come as a surprise, Miki-san, but Akemi-san and I have decided to set aside our conflict, and work together." And indeed, it probably was a great surprise to the other girl. Mami didn't tell her the reason why; that might lead to Sayaka asking about the details of their alliance, and Homura should be the one to explain that.