I have a custom Carvin doubleneck, 6-string guitar (with tremelo!) on top and 4-string bass on the bottom. All black, both with inline headstocks. I bought it in 1989. It was the only Carvin I'd seen in that configuration, with the guitar on top. The inline headstocks really work well together, too. All the tuners on both necks are almost in one line, and all easily accessible from the top. I found it hanging in the Hollywood Carvin store, a cancelled custom order for someone. As soon as I tried it, I liked it and bought it. It was the first and still the only doubleneck that I found reasonably comfortable to play. I still have it, although I rarely play it these days. If I remember right, I paid $1100 for it.
I tried a lot of doublenecks before finding that one. I didn't like any of them that had the bass on top. Two reasons: When playing bass, the back of my left hand would always be hitting the headstock of the guitar. When playing guitar, it was bending my wrist too far when playing chords. I didn't have either of those problems with the bass on the bottom.
The one silly thing on the Carvin was that Floyd Rose tremelo on the guitar neck originally had the standard arm, which really got in the way dangling down over the bass. I made up a new low profile tremelo bar that stays down about the level of all the strings. That solved the problem, and even served somewhat as a thumb rest while playing bass.
I also didn't like the original pickups in it (this was a 1988 instrument, back when Carvin's pickups were kind of lame), so I put DiMarzio humbuckers in the guitar and EMG's in the bass. It's still a nice bass/guitar to play. If I remember right, it weighs around 11 lbs.