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I have an old 12-string Harmony H1230 which has an unusual (?) double saddle:
More photos at http://harmony.demont.net/guitars/H1230/39.htm.
The Harmony catalog description, at that site, says "...dual-saddle bridge..." The strings are set deeply into the lower saddle and ride somewhat on top of the closer, shorter saddle. What is the mechanical/acoustical point of the dual-saddle?
Based on some discussions about building a 12 string at a Luthier's meeting, I suspect that the dual saddle addresses a couple of issues with the string placement and saddle forward tension.
On 12 strings that use bridge pins, the close sharp angle into the bridge helps keep the strings in place across the saddle. With a longer tail on the strings going to a tailpiece, the aligned force pushes the strings inward. Slotting a single saddle on a non-floating bridge isn't usually recommended for guitars, so adding a second saddle for the slots allows the string break across the main saddle without the sideways tension.
Another issue with the 12 string is that the extra set of strings adds more forward tension on the saddle, which can cause warping or breakage of saddles that are set high. A second saddle that takes some of this tension could distribute the load more evenly across the bridge.
Edit: although looking at the pictures I'd guess the second saddle is primarily for string alignment.
The reason for the second saddle -- with the slots -- is to provide string spacing and pairing. The string harp cannot space the strings correctly so a second saddle is needed.
My suspicion (a guess really) is that a regular saddle and pins bridge will locate the strings sufficiently that no other guide slots rare required, but with the combination of 12 strings and the tailpiece hanging from the bottom of the guitar, the strings would move more than would be desired (potentially touching when played?), so the second slotted saddle is required to positively locate the strings at the proper separation.
I think the Skinny Peacock does it for me. Exactly what my hunch is. I think the designer used some dedicated thinking on how to address various aspects of 12 string guitar construction:
The 'inner bridge' is shaped for intonation only, with little or no string groove, the ideal configuration for any chordophonic instrument (eg: PRS wrap-around bridge). The 'outer bridge' (rearmost) has the slots to ensure string spacing and negate the lateral string break-angle created by the tapering angle from the trapeze tailpiece.
Depending upon string guage, there exists upwards of 250lb of force, when a 12 string is tuned to standard pitch. That is a lot of tensile force and on a traditional 12 string, with pin anchors, that force is attempting to pull the bridge plate up and forwards. This is the main reason why acoustic 12 strings are prone to 'bellying', where the soundboard is warped upwards, throwing-out string action and thus intonation. [note: all traditional acoustics are 'flat-tops', there should be no arching visible on an acoustic guitar]. One way of alleviating that tendency, is to equip the instrument with a rear-block mounted trapeze string anchor/tailpiece, thus transferring that tensile force to the rear block, this in effect has the opposite reaction, as the string tension has a tendency to push down on the bridge/plate/soundboard thus increasing resonnance and negating any tendency for sounboard bellying. The second bridge therefore would also assist in that endeavour.
The warping/bellying of acoustics is something I have considerable experience with, as I live in an aluminium box in Cornwall UK, where it is very damp, most of the time. As a result, all my large steel-strung acoustics are showing signs of bellying, especially my 12 string. On my Yamaha Jumbo(6), I'm experimenting with a cantilever system, that counteracts the tension, by use of a fixture on the inside of the bridgeplate, transferring that tensile force via a rigging screw (bottle-screw tensioner) affixed to the forward neck block. This idea was inspired by the JLD 'Bridge Doctor' although my design uses tension instead of compression to solve the problem.
The 12 string is being fitted with a trapeze tailpiece, as seen in this Harmony, so I might as well adopt the same two-bridge configuration too.
The roundback acoustic, being mostly plastic, is not bellying so much, but it's not flat either... I'm toying with using a form of string-thru to the rear, but on an acoustic with a plastic bowled body that poses another set of challenges.
Finally, I must say that Harmony guitars are well respected in the folk circles of Kernow and the UK. They have beautiful resonance and that 12 string of yours Topo is exquisite, you are a lucky person to own such a delightful instrument ;=)
If I had to hazard a guess, it would be that the maker of the guitar realized he or she could transfer more energy to the sounding board with the addition of the second saddle.
The saddle further from the frets doesn't change anything tonically, since the mode of vibration is defined by the closer saddle. But it's imaginable that if the sounding board is vibrating significantly, there would still be some energy available between the first saddle and the tailpiece.
Even if this didn't increase the volume, the maker may have decided to do it just because it changed the sound of the instrument without degrading its pitch-performance. Any change is a candidate for that unique characteristic that makes your instrument the one everybody wants to buy!
EDIT: Another possibility is simply that the saddle that the maker was using had only ever been used for six strings before... perhaps he or she was worried it would break under the tension of 12 strings. So, divide those forces in half by using two pieces and no more worries!
My answer may be completely erroneous, but I know that a deep bridge groove for the string has a dampening effect on the string, and I know a deep bridge groove also provides more stability for positioning of the string which might be required with paired strings coming out of a trapeze. It seems logical that a second bridge with no deep grooves would allow the strings to ring free much more like a fret, an attribute which may be appealing to twelve string artists and fans.
