Welcome to the long road of micing guitar. You have two good mics, that's a good start.
First you need to know that the frequency responses of the two mics are complementary: the 57 will give more midrange, while the MD-421 will give you lows and highs. Blended they should give you a full spectrum. I'd reccomend placing them at the same distance from the speakers to avoid phase issues.
As far as the exact mic placement, well, that's where it gets interesting. Moving a single microphone's placement or angle as little as 1 cm or several degrees can introduce a drastic change. Closer to the cone gives you more treble and presence, while moving further out from the cone and/or increasing the mic angle removes treble / presence but increases fullness of the sound. It's finding the exact place for that guitar and amp with those settings through that particular cab that is the challenge.
What you'll want to do is:
1. Get a pair of high-quality ear-plugs of a kind that reduce loudness but keep a flat-frequency. Dynamic mics need a good loud source to sound good recorded (your pre-amp will thank you), not to mention that your amp will sound better cranked.
2. Play your amp loudly, bending down and getting your ears close to the speakers. Move your head around, noting exactly how the direct, close-mic'ed sound differs from what you're hearing in the room. Doing this you should start to notice that certain spots sound different right away.
3. Tweak your amp so that it sounds good from this angle, not necessarily in the room itself.
4. Find a rough position that would work for each type of mic (again, 57 emphasizes mids, and 421 emphasizes low-end & treble).
5. Now start experimenting with actual placement. While playing, have someone else (with ear protection!) move the mics while you listen with isolation headphons (preferably in another room isolated from the amp's room tone). Very slowly start moving the mic around until you've found the perfect placement point. Repeat with mic 2, checking how it sounds with the other mic. Again, if the mics aren't equidistant from the speaker you're also going to need to adjust for phase issues.
The last step is what took me the longest, as I didn't have a helper, so I needed to move the mic a tiny amount, record a clip, listen to it played back (both in isolation and in a mix) to see how it sounded, and then go and move the mic again. This was incredibly painstaking with just one person, so if you can bribe someone to help move the mic while you monitor it will save you hours if not days of time.
Of course, if you change guitars or amps or even the amp's settings you'll need to start the process over from scratch. With the Triaxis at least you can store the amp settings for each situation in a patch.
Hopefully you have a good deal of free time before you start recording next month.
I'm not sure about the 5 MD-421 settings, as I haven't gotten one yet. I've lately been using a single Audix I-5 (which I think gives a good middle-ground response between of the 57 and MD-421).