I know fashion is no good reason to buy or not buy a guitar, but the double neck is presently way out of fashion and unless there's a resurgence of classic progressive rock a la Genesis or Yes (about a million to one against, I'd say) it's hard to see that the double neck will make a return to popular acclaim any time soon. There's nothing inherently wrong with them of course, if two necks is what you need to get the job done then who am I to stand in your way, but unless you do genuinely need one I would suggest that you would just about always be better off with two different guitars. I did have a hankering a long long time ago to own a double-neck Ricky (bass and 12) because the bass player in Genesis used one and they were a great band in the early 70's but fortunately for me Peter Gabriel left in 1974, Phil Collins got into the driving seat, the band fell into the toilet shortly thereafter and as a result I went off the band and so decided against. Lucky for me, it would have cost a fortune and I would probably never have used it anyway apart from in my bedroom. And the damn things weigh a ton, even the lighter ones. So for me, a double neck is not on the cards any time soon.
Mick