This is just a guess but I am guessing you where shooting in RAW mode. I am not a mac person. But in windows if you shoot raw you will see the files in windows explorer as NEF files but if you using image software that doesn't support the raw files you will not see them unless you shot and use jpg format. May be the same on the mac.
If you set the camera to write to both cards as I do. I like the safety of writing to two separate memory cards. The NEF files and the video files are recorded to both cards. I have never found the need or a good reason to split the recorded data on two different cards. I use 2 16 gig class 10 cards. SanDisk. This way if one card goes bad I have a second card so I don't lose any recorded data.
I have never filled up a 16 gig card in one day. If you do, then dual 32 gig class 10 cards should be enough.