I have a digital Tascam 8-track recorder that I plan on using for recording a demo that I've been working on. I realize this is probably an elementary question, but I would just like verification. I noticed I can activate 2 of the 8 tracks at the same time for recording (where the track blinks red). Does this mean I can record my guitar track just once and have the song sounding like it has 2 guitars going at once. This would make the process easier, but would I lose any fidelity or fullness doing it this way or should I just record the guitar part twice, one track at a time?

Record it twice. If you record it once onto two tracks, then it essentially be the same as recording it once, only louder. Recording it twice will make it sound a lot fuller and just better.

are you just going to use one mic but have it recording to 2 tracks? cause that's not going to double your guitars like in a proper recording. it'll actually magnify all the shit you don't want... like hiss, noise, just general mess. it won't sound good at all.

if you are using two separate mics on the same cab it might be okay but if I was doing that I would just be using it to mix both tracks down to a single track and still would record a 2nd guitar track.

Unless I'm missing something, this actually means you'll be recording in stereo rather than in mono, which unless you have any particular stereo effects in mind, you don't need to do.

that's not what he would be doing. it's not recording in stereo.

Essentially what you'll have is just the exact same thing recorded twice, and played on top of each other. This won't really be discernible from having two separate guitar tracks, in which you'll get more nuances and each track will be (unless you are a total robot) slightly different and will sound more like what you are attempting to achieve.

NO. Record it twice; plain and simple. Unless, you're looking for an extremely perfect sounding recording, which a demo is not; then just change up the EQ a bit on the second track. I have the same Tascam as you, it works very well for demos/quick recordings, but if you ever want to release an EP or album I definitely recommend getting something like ProTools.

*EDIT* I also hear that Reaper and Cubase are really good DAWs