Two mono tracks sound better than one stereo track... why?

Theoretically, it should sound the same ...right? 

That is a good question. I have experimented with stereo miking of guitar using 2 mics and I do record into 2 mono tracks when I do this. Most of my guitar recording has been recording the same part 2 different times into two different tracks. That is doubling and by spreading the panning wide it also creates a really wide full sound. When done right this can really fatten the sound as well as give a nice full stereo spread. The parts generally have to be extremely precise to pull this off. Strumming stuff, like what you hear in country or rock music can be less precise. I generally only do this on the acoustic strumming stuff. (listen to some of my music to hear this) And it's not really loud in the mix... more of a background thing.

My only thought/guess (and I defer to those who really know the answer).... is:  As you stated, the two tracks seem to have a louder, fuller, volume than the same thing in a single stereo track. You said you compensated a bit for the volume increase whereas the stereo track did not seem quite so full and needed no compensation but was weak as a result.

Here's my guess

In the stereo track no matter where you set the pan.... straight up or panned at some other degree from center to fully right or left, all you will ever have is 100% of the signal. 

However, it seems reasonable to me that with two tracks mono.... you are able to have 200% to start with, and by panning something other than both straight up the center (200%) you can get lesser levels but you will always be somewhere north of 100%.

This gives you the cumulative effect of the tracks. More is better sort of thing. For tracking acoustic instruments this is a perfectly legitimate way to record. 2 mics into 2 mono tracks. 

You can however, use compression and other "mastering tools and techniques" to get the level of the single stereo track up and sounding fuller.  Capturing guitar with mics is an art unto itself. 

hope this helps a bit.... I would do whatever sounds best to you. There are no rules, only suggestions. The main thing is to keep the tracks nice and clean, not "in the red" and from that point, let it roll. 

When you finish, please do post it in the songs forum. I'd like to hear what you are doing.