I am not sure your question means anything. The Arc doesn't send any multichannel format to particular speakers so couldn't send one format to front speakers and another to rear speakers. 

If you send lossless TrueHD to the Arc it will send lossless audio to all the speakers in the Sonos HT system.

Well it must mean something, because you answered it perfectly. 
 

Thanks for your help. Appreciate it. 

Perhaps I was being pedantic.  Once the Arc is sending out individual channels it is not in any multichannel format, it is just a single stream of audio to each speaker, taken from the source.

I heard what you said, but I knew what you meant. :)

Thats great news. Makes the ARC much more valuable. 

Thanks for helping me out .