Shame they put the bass on top - that just seems wrong to me.
For right or wrong, doubleneck designers put the neck they think will be used the less of the two on top 99% of the time.
It also makes sense from a balance perspective to have the longest horn possible on top, which would normally be from the longer of the two scale length instruments.
I’m looking to buy a doubleneck with a bass on top and 6-string guitar on bottom.
......
Or – if anybody has any drawings or anything that can help me build my own, that’d also be really helpful.
You're going to have slim options for finding a new, premade instrument. They're very seldom made, for a lot of practical reasons.
One is that bands usually don't sound good when the bass player stops playing bass and starts playing guitar unless someone else steps in to take over the bass role.
The second is that most people don't play bass and guitar through the same rig, so it would need separate outputs (which few designs include) or an external switching system leading two amps. Which is a ton to carry to a gig, along with a very heavy instrument. However, modern modeling and IEM systems would make that much less of an issue than it used to be.
If you're going to design and build your own, here's my advice: Get two instruments you like, chop them up and glue the bodies together. Make a to-scale cardboard or plywood mockup first to triple check everything.
Another thing to consider is non-parallel necks. That would let you put the bridges as close together as possible, to reduce body size and weight, while keeping the nuts far enough apart to be able to play comfortably.
