Flying in the face of everything that's been said here... if you already have a pair of SM81's, I'd be tempted to get a decent multi-pattern LDC and then play around with various recording methods until you get a 'bingo' moment. THEN go spend some proper money. With two SDCs and one multi-pattern LDC you can try:
- Spaced pair of SDCs
- X/Y SDC's
- ORTF (horiz and vertical) SDCs
- Mid/Side with one SDC and an LDC in figure 8
- One SDC pointing at the 12 fret, one LDC coming over your right shoulder
- One LDC close, up the neck, one SDC near the back of the guitar pointing at the bridge
- One SDC up close, one LDC further out in omni to bring some room
One other thing to consider with two mics; phase issues. Nothing will kill your tone quicker.
But, at the end of the day, and as many others have said, it's down to experimentation, personal subjectivity and expenditure. It seems to be an area that can't be skimped on from your fingers to room to mic to preamp to A/D converter (assuming you're recording to disk). The way I did it; I read to the ends of the internets... and then went and rented a bunch of mics. One of the things that surprised me was the much-maligned KM184* turned out to be one of my favourite mics in MY space with MY guitars and MY playing. Your mileage may vary, of course. Once I'd played around and discounted some, then I went looking to purchase. There are deals to be had out there. Bear in mind one other thing while you're researching/experimenting though; there are some great little mom 'n' pop microphone makers making some great mics out there that represent great value to the home/project studio recordist and they are well worth looking into (but bear in mind, if you go through a buy/sell/buy phase that the "name" brands might hold their value for resale better than the smaller shop brands.)
So, right now I have a pair of 414 XLS's, which I can use for various tasks (including Blumlein stereo), a pair of 184s, a modern U47 type of tube LDC and a Royer 121 ribbon. I can do most everything I want with whichever guitar I pick up with those... although, I'm probably not 'done' yet. I'd like a pair of tube SDCs, some more flavourful ribbons... (of course, one may never be 'done').
* the KM184 whenever mentioned on any forum, anywhere, will always be followed by 'it's not as good as the KM84', which is the mic it replaced in Neumann's line up. But guess what? You can't easily GET 84s so it's just an annoying–and some might argue–self-perpetuating internet 'truism'.