On my one-slot bodies (7D and 6D) I shoot RAW and Jpeg Large, but honestly that's force of habit more than anything else since it's probably been a couple of years since I last used a jpeg file from the 7D, and I've never used jpegs from the 6D... I really should just change it to RAW-only on both, I guess...
On my 5D III I now shoot RAW + RAW, so both cards record the same files, simply as a card failure safety thing.
On the X-E1 I actually find myself using the jpegs from time to time so the dual file type recording is more handy there.
So you own both Mk3 and 6D? I've never seen someone who has both. If you have Mk3, why would you want 6D? Just curious.
I think he use it as a backup or second camera. Primes on 6D and Zooms on 5D3 when shooting a wedding.