I would recommend against having a two hard drive solution, especially for traveling. Two hard drives are usually reserved for desktop replacements, i.e., the 17"+ boat anchors. If you want something built in, I think your best option as far as I know is the ASUS Transformer Book (Engadget's review
). Otherwise, just get the laptop and swap out the HDD for a decently sized SSD and put the HDD in a USB 3.0 external enclosure. You'll get near native performance and the space when you need it.
Otherwise, I hear Samsung's Chronos 7 is good if you need battery life.