Hey Sami, I admire your spunk for wanting to build/convert a doubleneck for yourself. I do that for a living. First off, unless you're willing to put in a lot and effort or suffer great dissappointment, consider just buying a conversion or custom unit. Check out www.ckdoublenecks.com . You can, of course, use the existing necks, tuners, etc. and all the supplies you need, such as nuts, tuners, bridges, etc. are readily available from firms such as Stewart McDonald - www.stewmack.com. You will not be pleased with the approach you are indicating.
Double your music - double your fun