There's no need to actually remove the card. Just set the camera to auto-switch. You get full performance while it's using compact flash.
I know that's the theory, but have you tested this personally? Thanks!
One of the first things I did when I got the camera was play around with those settings. I *really* like the idea of recording RAW to two cards, even though I've yet to lose even a single frame to a bad card. Nice idea, except that the buffer gets reeeeeeeeally shallow and painfully slow when it fills up. The original Digital Rebel might actually outperform the 5DIII with dual RAW recording. At least, it feels that way.
I eventually settled on the auto-switch setting. If there's a difference in performance between that and no SD card in the camera at all, I certainly haven't noticed it. Of course, once the CF card does
fill up, then you're back to shallow, slow, painful buffer...but only until you pop back in a fresh CF card. And, as I originally wrote, shallow and slow is way
better than a brick....
I did use dual RAW when I shot the annular eclipse over the Grand Canyon. I knew I wasn't going to fill up the buffer, even with several-step brackets. Other than that sort of thing, the only practical use I can think of for dual recording would be with an EyeFi card...and, nifty as that thing seems, I personally can't think of any situation where I'd actually use one.