The message formats of TCAS and SSR will be totally different, and the reply will be different. By totally different I mean there will be a firld set somewhere within the message defining what type of message it is. Be it an SSR interogation by Radar head or A.N. Other. I'm not 100% sure about this, but that's how it would have to work. Also the SSR will be expecting a response within a pre-set amount of time - the time foir max. range of the radar multiplied by 2 plus a bit added for processing. Chances are the time for A/C Bs response would be outside this. And i'm also assuming there would be some sort of addressing going on, at a very basic level. I.e the SSR head would have an ID, the 2 A/C would have an ID which is independant of SSR code. Don't quite me but that's roughly how I understand the 2 work.