The tension gain with the octave strings is massive. Just think that the D string on a 4 string bass (not the E or A) has the highest tension. And an E octave string regularly has the strength of a normal G string or more. On a 12 string you have all that doubled. Of course, with a medium scale tension is overall less and this guy uses very light strings (regular E is an 85 - that's what most of us use as an A string).
But still: My Ric 4003/8 really isn't built to withstand the tension of 8 strings, twin trussrods or not, and my 12 string Musicvox Space Cadet has a mammoth neck, stronger and chunkier than any 5 string or 6 string bass I've seen, and two trussrods to adjust. OTOH, my 8 string Les Paul Standard holds up fine to the 8 strings so Gibson's truss rod system might be sturdier than others (certainly more so than Ric's), I even have a (4 string) fretless Les Paul DeLuxe where the neck is so straight that the truss rod is completely loosened or action would be too low.
In any case, the mod is pretty hideous, he got carried away on that one. Guy was probably a Cheap Trick enthusiast, combining Nielsen's preferred Explorer look with Petersson's multitude of strings extravaganzas.
Uwe
