The reason there is a split in the memory is to do with USB mass storage mode...
Think of the storage in your phone as two devices.. hard disk and USB memory stick..
The Operating system and all the apps you've installed together with your settings, contacts, messages and calendar data is held on the hard disk.. Your videos, photos and music, however are held on the USB key..
The N9 has the ability to switch into USB storage mode.. in this mode, the "usb memory stick" portion of the internal storage is unplugged from the phone and connected to your PC (via the USB cable)
whilst this happens, your phone cannot access this part of the memory, on your PC can.. until you disconnect the phone and the USB memory stick is re-plugged into the phone..
Now, the issue with changing the size/ratio of the the hard disk storage (that is permanently attached to the phone) and the usb memory stick portion (that can be attached to the phone or to a PC) is that you will reduce how much storage there is for important data.. such as apps, email, contacts etc..
Now you might think that 8gb is more than enough for your apps and contacts.. but some apps can be huge (particularly games) so this is why the reserved space is so large..
You'll never be able to merge the portions into one.. they will forever be kept split in two parts.. And it is "complex" messing with the ratios.. there's lots of extremely dangerous sensitive steps to follow and you must be proficient with linux generally to be safe..
As grschinon said, you can use apps to "browse" the internal storage.. but you must be very careful where and what you do..
The internal storage that you currently cannot access is fragile to poking.. you could mess things up. Also, you will find the phone cannot access file data outside of the main storage.. for example, if you were to copy images on to your phone using filebox (or use filebox to move them) your phone's gallery will probably fail to see the images until they are moved back.. It is very particular about where files are kept..
Tricky, I'm afraid..
Remember, be careful, you can really upset your phone messing around with its innards
