Help need Help thickening up Acoustic/Vocal sound

Sambostar

Singer is high and the acoustic is high also.  I'm loathe to plug into an amp and add delay or anything for a small room in a pub of maybe 20-30 people, it sounds OK when I'm singing and backing, but it's way too much high register when it only the guitar and her, although her singing is great the guitar and her voice are fighting.  It's sounds OK on the Classical also, which is a fuller, richer sound with her singing, but as I bite my nails, it's not really loud enough.  Am I making sense?

I have no experience of these acoustic duet things and I don't really like steel string acoustics to be honest as they all sound jangling to me and prefer classicals. 

What should I do?  It's all there, but I can't sing along to everything to fill in the frequencies because for starters I ain't that good.  Been practicing in a big open room, deliberately to get it as dry and thing as possible in terms of worst case scenario. 

How could we thicken up the sound in general? 

Any advice much appreciated.  I'm not used to this and trying to sing, play and fill and keep everything together is enough to deal with, without worrying about frequencies and richness.

ICBM

Leave your strings on for much longer than you normally would. Old strings sound much fuller and softer - you have to keep them clean though, or they will corrode.

Use your fingers not a pick, if you aren't already - it gives a thicker tone. If you're not used to fingerpicking, hold your hand as if there's a pick there but just hit the strings with your fingers.

What guitar are you using? Some are very much fuller-sounding than others...

Sambostar

I think it's a combination of brand new strings, scratchy, brand new guitar and a parlour size.  Agree with the finger and I'm trying to add some bass notes in too below the chords.  The rest is fingerpicking however and some chordy stuff has to be plucked in unison to give it the right texture, like a piano/keyboard sort of. The singer is very high and I'm very deep, it kind of needs some smooth mids in there somehow, especially for the non folky, non classical songs, I wish my voice was a bit higher.

I'm being really critical as it's only second meet since we started, so only a few hours in and literally learning songs on the fly without any more thought to it than getting the stuff out.  Maybe it might be better with a dreadnought type guitar after all and an older one at that, but they are so uncomfortable.  New strings are horrible too.

I'II try playing purely with the fingers though for the pop rock stuff, see if it takes the edge off it.

ICBM

I work with the same kind of female singer with a high, pure voice so I understand exactly where you're coming from. We actually often use an old Eko Ranger guitar, with strings changed as infrequently as possible - it has a really thick, almost 'dead' by proper acoustic standards, sound to it which is ideal. (Especially as I don't sing at all, either high or low!)

My proper guitars are a Gibson Dove and a Gibson CJ-165, which are both quite punchy - the Dove is big and scooped and the little CJ is all midrange. Both work well with her voice on recordings but if anything the Eko is better live - it's also not as loud as the Dove, and she doesn't have the strongest of voices so the balance is quite good even when I thrash it, which I am prone to do...