<p>My understanding is that when a TTL flash is used, the camera reads the light bouncing back from the subject and, when enough has come in, turns off the flash. The camera cannot control any manual flash-guns used and must assume that all the light on the subject comes from the TTL flash. Therefore I said, in an earlier post, "you will need to use your TTL flash also in manual mode." I say again that the simplest and least expensive solution is optically activated sensors on both manual flash-guns.</p>