I have a Carvin doubleneck that I bought around 1988. It's 6-string guitar on top, 4-string fretted bass on the bottom, both with inline headstocks, all black. It's not that heavy, about 12 lbs. Both necks sound good, and feel and play about like the equivalent Carvin models.
Back in those days, I was really into Rickenbackers, and I was thinking about getting a Rick doubleneck. I tried a couple of the Rick doublenecks in stores, and I found them to be just awful, really heavy and clumsy to play. I didn't know much about Carvin then, and I wandered into Carvin's Hollywood store. And there was this black doubleneck hanging on the wall. They said it had been ordered by some rock star and then cancelled. It was unusual because the guitar was on top. I immediately liked it and bought it (about $1100).
I've found it to be the most comfortable and reasonable to play of any doubleneck I've tried. I like having the bass on the lower neck. For my left hand, there's more clearance. Playing the guitar neck up high bothered me less. A year or so later I modified it, putting in DiMarzio pickups on the guitar and EMG's on the bass. I even fitted a tremelo on the guitar, and made up a special low-profile bar for it. Pretty cool for its day.
The main purpose I bought it for was sitting at home in the studio, learning and working on songs. It was handy to have both instruments right there to experiment and try out parts. I don't think I actually ever played a gig with it, but I could have. These days, I hardly ever pick up a guitar. So the doubleneck has been sitting in its case for the last 15 years. But I plan to keep it.
If you are looking for a practical, reasonable doubleneck, I recommend that you check out Carvin. I think they still make them, on special order anyway.