I have done something like this. I got an Epiphone PeeWee It's a 19" scale. The are currently discontinued but you might be able to find one for cheap. I tuned it an octave higher than my normal. It goes from C#3-C#5 with a D'Addario .007 at the top. The tuners are the worst I've ever experienced it was difficult getting it to wind the string. My hypothesis was confirmed when I played, it doesn't sound that much higher than a regular guitar. If you slowly strum all the open notes, the interval between the 6th and the 1st is still 2 octaves. It would sound high if you still had a regular C#2, but the pitch relativity that you hear it still the same on a regular guitar. Your just lacking the lower note center. I think it could be useful in composition when you layer it on top of a guitar tuned an octave lower, it would create great contrast. You should be able to get an ESP F-Jr With it's 20.5" scale, all you need to do is change your initial tuning to C standard. Then you can have C2-C4 on one guitar and a C3-C5 on that one since the .007 dictates how high you may go. Because this mini guitar didn't meet my needs of adding a second higher string I ended up resolving this problem by shifting my major third. For example, you could tune an 8 string. BEADF#BEA (6 string B with two high strings) and I found it feels and sounds higher even though the pitch is the same.