D7100 buffer size vs. card speeds
Imaging-resource say in their preview that the D7100 supports the newer UHS104 standard cards which means the theoretical max write speed is 99 MB/second as opposed to the D7000 which is limited to 40.5 MB/second (using the older original UHS-1 cards). Tho if I remember correctly the D7000 could only write at about 28MB/second. So only tests will tell if the D7100 does in fact write faster than the D7000.
The D7100 will not write simultaneously to both cards with RAW files (unless you are using the second card as a backup I suppose, but this dosn't help here), but it can write RAW to one card and Jpeg to the other. I assume this is simultaneous as I understand the D7000 can do this. It has been suggested to do this if you are shooting RAW+Jpeg.