Yes. On the Nikon D7100, you can copy images from one card to the other in-camera. The camera lets you choose individual images or an entire folder, then select a folder on the second card as the destination. Copying a few hundred RAW files is practical, though it does take some time.

One limitation: if you repeat the process and copy the whole folder again, the camera detects existing files and asks whether to overwrite, but there isn’t a convenient “skip all duplicates” option.

That said, the main purpose of dual card slots is usually backup at capture time: keep two cards in the camera and set it to write both simultaneously. That protects against card failure immediately. If you’re also worried about theft or loss, the safest approach is to physically separate the second card from the camera after shooting.