Another noticeable double-neck guitar is this nylon string acoustic one by luthier Fred Carlson. Here the body is hardly bigger than a single neck one - and this is good. Though it has six nylon strings, the longer neck could be considered as a bass one since its scale is 34' - but the baritone label doesn't require a strict scale and baritone guitars can have various scales, starting from 27'... The strings make the difference, for example the famous Fender Bass VI (30' scale) can be a quite different instrument with bass strings or baritone strings.
The other instrument on the picture from the same luthier is a sympathetic guitar - called Sympitar - with 12 sympathetic resonating strings inside the neck.
18 strings, not bad, eh? (this is my 'the more strings the better' of the week).