Bass/treble balance wrong is exactly what you want I would think. And that's why you're using 2 mics on different parts of the guitar. You've got the whole spectrum (hopefully) on two dicrete tracks. Blend to taste. Each track by itself may not sound wonderful, but pan them left and right (10 o'clock and 2 o'clock are good starting points) and you've got something.
I personally use an sm81 to capture the highs, either about the 12th fret -or- below the soundhole pointed up towards it. (to avoid that little soundhole puff of air thing)
And then a LD on the soundboard just beyond the bridge. Usually from above. -or- A LD pointed parallel to the top pointing in from the end of the guitar. (from the end-pin side if that wasn't clear) I find that sometimes pointing the mic to pick up accross the top, rather than at the top, reduces boominess. (Don't forget the bass roll-off switch) No reason these methods can't be applied with 2 81's. In fact, it may work even better. (Don't know, I only have one 81) Good, solid mic choice BTW.
But, YMMV. Depends a lot on the characteristics of your guitar.
Mine BTW is a very tonally complex 20+ year old
Martin D35. If you have something similar, one of these methods should work well for you. As far as volume balance goes: That's why the preamps and mixers have those little buttons and sliders on 'em.
Getting a good solo acoustic sound will take some experimentation. Just an inch or two difference in mic placement sometimes changes the tone.
Have fun!
Tom
