I'm no expert on musical instruments, but is there any point for that one to have two necks? The idea with multi-necked guitars seems to be combining different instruments (such as guitar + bass), but the necks on this appears to be the same.
Most likely a real instrument built like this would have different registers (how high or low the overall range is), so like you said, possibly an alto register and a bass register; though with two 7-stringed necks, that thing would have one hell of a voice range.
As a hobby guitarist, here are some reasons why a two-necked guitar might be desirable:
A different set of string gauges, meaning one has thicker/heavier strings with a different tone. You can also put a different set of pickups to have more tone options. One neck could have a different tuning, especially if it's all tuned to a chord. This means you can just pluck the top neck as a drone chord while you solo on the standard tuning on the bottom neck. This means that Miku could play a chord progression by simply slamming a finger flat on the neck to hit a "bar chord", while it actually plays diminished chords, and solo appropriately on the other neck. What Gollgagh said is also potentially true, allowing for a more dynamic range.
There's also the benefit of plugging into two separate amplifiers, if all the pick-ups haven't been all wired together. It would sound cleaner than running a single guitar through a loop pedal for a chord tone to solo over. Also, they're not 7-stringed necks. That 7th peg you see near the tuning heads is supposed to be holding down the two high strings for tension at the nut. This makes it a little weird, because that would mean Miku's bottom neck is reverse stringed, meaning it's like having a right-handed guitar flipped over into a lefty, without changing the order of tuning, which is what Albert King did (who is known for achieving insane bends because of it).
So, actually, Miku is more badass than you ever imagined. But it's not plugged in so it doesn't matter. :P
