Are double-strings louder than single strings?

Assuming you're talking about an acoustic guitar, a 12 string does have more sustain than a 6 string. The reason that it sustains longer has more to do with the composition of the strings themselves than with the number of stings you have. A 12 string will also be a little louder but the volume of an acoustic guitar has more to do with the size and shape of the sound chamber than the number of strings you're playing. My perception of the audible effect of 12 strings over 6 is that the 12 string sounds brighter.

You must remember that a 12 string will be more difficult to play because you have to fret and strike twice the number of strings simultaneously. A 12 string requires more precision and twice the left hand strength of an acoustic 6 string. You'll also loose something in terms of action on the fretboard because the neck is much bigger on the 12 string.

The first thing you should experiment with are the strings you're using. If they're old, they won't sustain and they won't be as loud. If you're playing your guitar every day, you should probably change your strings every month or so depending on how hard you are on them. Try different brands and weights until you find the ones you like best.

If string changes don't work for you, start looking at acoustic guitars with concert sized bodies. Larger body size on an acoustic equals more volume, much more quickly than additional strings. In any case, you'll need to visit local music stores that have guitars. This is not something you can do online. Shopping online has it's place but picking out a new guitar isn't one of them. You can't hear and feel a guitar listed on a web site. Every guitar is different. It doesn't matter if it's the same make and model made on the same equipment. There are environmental, material and human factors that introduce variables detectable by a player. However online pricing my help you get a better deal from your local retailer.

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The C and the A are the ones that are doubled. They are usually tuned an octave apart. This gives a much fuller sound than a regular 4 string. The basic tuning is still the same as any ukulele, NOT like a guitar.