Hi, I record my band a lot in whatever the room is that we’ve got at the time. I don’t think you should stress over the sound of the room unless it sounds bad to you while you’re playing. Then, as Stephen says, bring in the upholstered furniture and blankets until you’re happier with the sound you’re hearing while playing. After a few beers!
Another way to use the two SM58’s is to put them both front and center in an x-y pattern. Takes care of any phasing issues, and still delivers a decent stereo field. That’s how I would do it. If you use the spaced pair, remember the 3-1 rule: mics have to be three times farther apart than either of them is to the sound source, to reduce phasing issues.
If you’re going to buy more mics, that will help assuming you’re interested in a more studio-like recording. I don’t know what sort of interface and inputs you have, but in my case, with guitar, keys, bass, and drums, I use 7 inputs and record 7 tracks - 4 mics on the drums (2 overheads (always equidistant from the center of the snare top), one on snare, one on kick), one mic on the guitar amp, and then I take a direct input from the bass and the keys. My mics are real cheapos, SM57’s and two low cost large diaphragm condensers, which I use as overheads. There’s lots of bleed from one mic to the others, but that seems to help mush it all together.
Doing it the way I do it requires you to address mixing it all together to produce a stereo file. Which is a completely different topic. If you want to hear any of the stuff I’ve done as I have described, I have posted recordings on the “made with cubase” forum.
If you have line inputs, you could feed the electronic drums and the keys into your interface, and that takes the room out of the equation for the tracked sound of those (but these sounds will still appear in any other mics you are using). You are then tasked with adding “room” to these using reverb/delay plugins. That’s what I do.
But if you’re just trying to capture the spirit of the moment, without getting too crazy, I’d say go with the x-y, and SM-58’s will do well enough for that.
The best thing about once or twice a month jam sessions is, you have them!!! How great is that.