There's an app called
Root External 2 Internal SD that swaps the memory spaces (so you can use the SD card as "internal" memory ([I've never tried it, since I have the same amount of each, so it wouldn't do anything for me]), but I've never heard of an app, or even a ROM, that combined them. SunOS used to be able to combine hardware into a single "storage", but *nix considers each device (internal storage, SD card) as a separate storage device.