One reason to record a mono instrument like a guitar in stereo is so that you can later adjust the timber of the instrument via levels.  I usually record an acoustic guitar either X-Y between the 2 hands of the player (look up X_Y microphone placement on google) or spread, with one aimed at the "pegs" and one aimed at the 12th fret of the guitar.  Fret side is lighter and airier, the "peg" end is deeper and crisper on attack.  Then you can mix the two via levels, compression and pan to get the sound you want.

Stereo micing a voice is of little benefit, but recording with two mics set different places can give you options for later mixing with the proximity effect of close micing or the airy sound of being further away.  Just be careful if you try to use both, combining the two signals can cause some nasty comb filters that make some frequencies disappear while others will be boosted and make mixing consistent levels near impossible at times.

As for setting it up in Sonar, I usually use 2 mono tracks so I have maximum flexibility.  If you record to one stereo track, you can only process both sides the same.  You do have the option of "split to mono" but their is no real gain to not doing it this way to start with.

Understand though, a single voice and a guitar are both "mono instruments" with the spacing of a guitars strings making a stereo recording sometimes advantageous.  A single voice does not really benefit at all from stereo except for every live environments where you are actually recording the room in stereo more than the voice itself.

frkfir
Sorry, but I don't quite understand what you told me to do.
I am new to this software :(

A picture is worth a thousand words.
 https://vimeo.com/149988041

If I want to see the waves appear in both sides of a stereo track, for my device (NI komplete6 audio)I have to select the input on which it is plugged in as a mono source.I think the logic there is that if you choose it as a stereo input and there is no signal on one side, that's what you get. A stereo recording with one side beinig silent. Seems counterintuitive but that's how it works for me.

A stereo signal is more than just the same mono signal cloned to the other side - it contains subtle (or not so)changes, typically the time differential from entering each ear (or two microphones not occupying the same, exact space).  That provides your brain w/ the information to figure out which side of your head the sound is coming from, among other things.
 
So it isn't counterintuitive that a mono signal should just be a single signal when recording.  But if you look at the signal in your output meter in SÖNAR for that mono recording you'll see that the mono  recording is output as an exactly matched (in volume) stereo pair and shouldn't come out of a single speaker, but both.  That is how you pan a mono signal to whichever side you want it in the stereo field.  You aren't adding the signal to one side - you are taking it away from that side of the stereo field. 

In addition to the above, in the off chance that the "new to this software" implies new to DAWs in general, Bitflipper made a really nice post a couple years ago worth reading. Mono vs. stereo unto itself can create some long discussions, so your initial question may have just nicked the tip of the iceberg.

Something this thread hasn't touched on, or not in detail at least, which might be useful to the OP, is that there's almost never a good reason to record a guitar in stereo. I think I'd like to go back to the start and find out why the OP is trying to do this.

As the OP is new to the software and therefore perhaps recording (given the question) it's also worth pointing out that.....
 
Mono = identical signal in both left and right channels
Stereo = material that is different in left and right channels
 
You'll never get a true stereo track from a single mic source (prior to any processing). You can of course convert the mono track to stereo track but of course the material is still mono.

John T
Something this thread hasn't touched on, or not in detail at least, which might be useful to the OP, is that there's almost never a good reason to record a guitar in stereo. I think I'd like to go back to the start and find out why the OP is trying to do this.

I was thinking the same. My guess is, it's the very, very common misconception among beginners that every track needs to be stereo because you're making a "stereo record". It comes as a surprise to many, that usually most of the tracks are mono.
To the OP: Recording a mono source as mono leaves you the options to pan the instrument wherever you want in the stereo field.
A stereo track, one that has different material left and right, can not be panned as easily, as the panning is actually balance setting. Then you need a plugin like Channel Tools to pan it in a controlled way (ie. not fading away the right side material when you pan to left). Piano is a good example: If you record (stereo) the right hand parts on the right channel and left hand parts on the left, when you pan it to one side, the material on the opposite side does not move to  the panned side, but simply fades inaudible.

Hey guys,
 
the person who asked the question disappeared, not telling is any advice worked for him...
 
I have pretty much the same question. I read and tried more or less all that you wrote here.
 
I recorded voice through a mic connected to TASCAM US2*2 in the IN1. Voice is heard only in the left speaker.
 
1. Trying to clone the audio track and then panning left and right didn't work: the upper line (left) in the cloned track is still in the upper part and no matter how I pan - the sound comes from the left speaker.
2. Instead of the "convert to stereo" right click gives only option "convert to mono"
 
Could you tell if I have any chance to make it sound from both speakers?
I want to record a song, I have only one mic (I have never seen a singer singing in two mics in the front on him to get a stereo sound :-) ). What's the problem?
 
Thank you!

Which version of Sonar ? check your routing on your Tascam.
load a stereo file in to sonar and pan that stereo file left to right. make sure you have sound in L + R speakers.
you need to eliminate the Tascam as a problem then you can go on to other things.