I get pretty decent results from a Marshall 603 and a Sennheiser MD421. I almost never use two condenser mics for recording an acoustic instrument, it's always a combination of a dynamic mic and a condenser mic. I will also record the pickup if it's not a Fishman.
As far as placement goes, I find that there are some general things which work but it tends to vary with the guitar, player, and room. Because of that I always have the person play for a couple minutes while I cover one ear and listen at different points with the other ear to see which places sound the best.
While I'm doing this I'm considering how my mics respond to different things, and how I can best use this to obtain the sound I'm trying to get. For instance the 603 has a nice off-axis response, which is good because it has such a wide pickup pattern. So I like to try using it from above, maybe over the guitarist's shoulder or something, rather than right up on the guitar. The MD421 has something like five levels of low-end rolloff so I'll do the opposite with that and use it to get the "punchiness", and roll off the low end as needed to avoid boominess.
It's important to note here that you should spend some time with each of your mics recording different things in different ways so you know how they respond. It's a lot easier to get the sounds you're after if you know that Mic A records a certain range of notes beautifully but is mediocre with other frequencies and has a weird off-axis response, while Mic B is solid across the board but doesn't stand out at all.
Phasing issues will play a role in multiple-mic recording of a single sound source too. If you're not familiar with the hows and whys of phase problems with recording, Google is your friend. To combat it, set up one mic first and then put on headphones and move the other mic around while the guitarist plays. Make sure you're running mono into the phones. Phase issues will cause the sound to be muddy and lack low end, so it's fairly easy to tell when you've got it right and when you don't. Some audio software and some mic preamps will allow you to reverse the phase of your signal, which is a handy tool which you should use if you can.