Old truss rods can be trouble because the nut can rust or otherwise get stuck. Exerting too much force to loosen it can bust the rod or strip the nut. Appropriate lubrication can help free it, of course. On a newer guitar, adjusting the truss rod isn't particularly daunting as long as you have some idea what you're doing. Tightening the rod (a clockwise motion on the nut from the headstock end) will remove some concavity; a counter-clockwise motion loosens then nut and allows the strings to induce more concavity. This really shouldn't be done in place of other adjustments but it definitely has its place. The three adjustments that influence action are nut slot depth, saddle height, and (indirectly) neck relief (which is what the truss rod is all about). If the action is really off, a truss rod adjustment isn't necessarily (or even likely) the right remedy. If the neck is pitched forward, the saddle is the wrong height, or the nut slots are too shallow, making the fingerboard less concave might help a bit at the middle of the board, but it's really not a cure.
Regarding Guild's double truss rods, a lot of people seem to lament their passing. Not me. The new Guild 12-strings have slimmer necks with a single truss rod straddled by carbon fiber bars. It's a very rigid and more manageable assembly and some of the horrors of incorrectly adjusted adjusted dual rods (like neck twisting) are obviated by the single rod. Guilds of old are known for resisting the need for a neck reset more than most other 12-strings, but remember -- neck resets aren't necessitated by neck bowing but by changes in the body of the guitar around the neck block that change the angle of the neck, not its curvature. Think of the guitar body as becoming slightly bean-shaped with the neck block and tail block both moving up and toward the sound hole a small amount. Guilds did better than some competitors because they were built like tanks (in a good way). Only time will tell, but the new Traditional Series Guild 12's look like they'll do just fine under full tension. In the meantime, they're great sounding and great playing guitars (although a wider nut wouldn't hurt).