With a 12-bit 15 MHz Analog->Digital Converter, we will need to read in 180 million bits per second, or 21.46 MiB/second. With two cards splitting the load, each of them would need to be able to write at 10.73 MiB/second.
From start to finish, it took 13.467 seconds, or 10.88 MiB/s which is too close to the minimum to be reliable. Future runs were slower. For example one was as bad as 14.59 seconds (10.04 MiB/s). Another test yielded 14.06 seconds (10.42 MiB/s).
Without using the SD card's pre-erase feature (which I can only use for the first 4 gigs on the SD card for some strange reason), I am getting 15.06 seconds (9.72 MiB/s).
EDIT : with an optimization and no pre-erase I am now getting 14.2 seconds (10.3 MiB/s).
EDIT #2 : I just tried doubling the data written. Now I am getting 9.83 MiB/s.
NOTE : It looks like the reason for these speeds is because I am using this kind of SD card. I've found that bigger (physically) and smaller (capacity) cards can write faster.
Conclusion: Ok, so two SD cards are almost fast enough but not quite. Incredible. I was so close! I'm so close now that I will probably just buy another SD card. But dang.. 3 SD cards! This project is broadening into gargantuan proportions.