I've got one similar, not for sale though.
It's the washburn bantam double neck, but the lower neck is a 6 string guitar. I use it at home for brainstorming song ideas, that way I don't have to un-plug\re-plug instruments.
Its a really decent instrument with a good sound and easy access all the way to the 24th fret, something you don't get on a lot of basses.
It has some downsides though, the biggest being the stringing of the bass side. The ball end goes into the bridge. the other end feeds under 1 of 2 clamps just past the nut. Each clamp holds 2 strings, so you have to put on your E and A string together. While pulling them pretty tight, use a hex key to tighten the clamp.
Hopefully you pulled them sufficiently tight so you can then tune them with the adjustment knob on the bridge.
Then clipping the strings to flush is just about impossible.
Additionally, if you break 1 string you have to replace 2
