For starters, just because an amp has two speakers doesn't automatically make it 'stereo'.
Often, it is determined by what the amps designer wishes to do with the amp. How they want it to sound. A 2x12 (two 12" speakers) will sound different than a 2x8 in the same amp.
Also, you'll usually only find two speakers in amps with higher wattages, though this isn't a hard and fast rule.
Two speakers will move more air, thus making an amp a bit louder for a given wattage. It can also result in a cleaner Clean Channel at higher volumes. This goes back to what the designer had in mind.
I'm sure that you've seen amps advertised as Stereo? I've never tried one, but I think that term is a bit misleading no matter what they do with the electronics.
To me, Stereo is a true seperation of Left and Right. I can't see two speakers that are four inches apart being anywhere near stereo.
I get stereo by running into two amps placed at least ten feet apart. Now I hope this doesn't confuse you
My signal is not stereo, yet. It's the same dry signal going into two amps. So far, the only difference is in the setting of the Tone Controls on each amp.
At the end of my Effects Chain (bunch of boxes on the floor) is a Stereo Phaser. This box has two outputs. One goes Left, the other Right. When the effect is turned On, you can literally hear the volume swell from one side to the other, or the Effect move back and forth. It depends on the Settings that I dial in on the box.
I think whoever decided to call that 'stereo' was just being lazy. There has to be a better term for it! It's still only one instrument. We can't think of it in the same sense as L&R on our audio systems.
For optimal stereo listening; if two speakers are ten feet apart, the listener is ten feet back. Correct?
To be honest, there's only one tune that I really even use it for. Billy Thorpes Children of the Sun
. Even then, it only sounds good close up. If I were playing at a party in your backyard, and you were 40 feet away at the volleyball net, you wouldn't even notice the seperation. :'(