Regarding Home Studio and usb mic level

The preferred driver standard is usually ASIO but you can't use more than one interface with that normally. So that would either be the Tascam or the Samson. Which is dumb but honestly USB mics are dumb, I'm sorry to say. I hate that they market these as "professional". If the mic also has XLR out and the Tascam offers +48V Phantom power the best route is probably to hook it up to the Tascam simply so you can use one driver/interface for both recording and playback. That allows you to use ASIO which will generally give you the lowest latency (the time difference between saying/singing something and hearing it back). Any speakers or headphones would need to be hooked up to the Tascam as well.

If you have to use the USB connection for the mic you will need to use some other device for playback (some mics do have headphone outs, you may be lucky), experiment with different driver modes to see which will let you select multiple devices. It depends on the devices you have. MME will for sure let you use them all but you'll get the worst latency by far.

As for the stereo track imbalance, honestly I wouldn't even expect the mic to be stereo? Almost no mics are, usually stereo means "two mics" as the entire point of stereo is to capture two different places in a space. I'm sure it can deliver a stereo audio stream but I don't think there would be any difference between the two (except for level, in this case). It may be that it offers a hotter and cooler version as a matter of convenience. Does the documentation say anything about it? In any case I would pick either the left or right side and record on a mono track because you're just wasting disk space and resources recording in stereo, there is nothing stereo about a vocal recorded by a single mic. It'll automatically become stereo when you slap a reverb on.

I couldn't remember what her I/O was! I hadn't seen it for 4 years LOL Tascam iUR2 and there's a breakout cable for the xlr input. She bought the usb mic for podcasting as recommended by Trey Ratcliff, the photographer.
    I see it as a "white elephant", mostly.
RE the stereo.    That's what I told her, but need to convince her of. I hope there are updated drivers for it. It isn't even listed in Tascam's legacy stuff......
 
@ BobF   I didn't think of that I went to her onboard Realtec, but will go through that all again.
 
Thanks guys. So, the Wasapi is not an option? She has Windows 10 Home on the same desktop PC as mine (see signature)?
Michael

Was looking for more info on the Tascam iu2. Seems all Tascam links lead to dead pages. Found this post from a guitar forum 2013. Bottom line, no drivers and certainly unsupported for W10. 
 
Myself I would avoid the Tascam IUR2, gets poor reviews on Amazon and its now considered a legacy Tascam product that micromanager Henry J of Gibson will veto placing any funds on further development.  (I.e No future Win /OSX drivers.  Add its known to be buggy with iPad too..

And the stereo thing, With on board audio and possibly USB drivers for class compliant audio, you don't get a mono option from what I see. Only the stereo. 
It's expecting a stereo signal. 
One option I've used ( was desperate ) was to use a small mixer and the RCA output to a TRS mini jack into the computer. It actually worked just fine and was not bad as far as noise floor goes. The noise floor does depend on the sound card involved. 
 
She can use the mike, but record into anything but Sonar. Otherwise might as well use built in mike on monitor :) 

I believe she got the mic, a Samson CO1U(hopefully that's better one), 2 or 3 years ago and then put it in a drawer, and  I ordered the Tascam iUR2, then. So, there's no returning either. The podcast phase is in hibernation...
No drivers for it on the Tascam site.
Replacing both is my plan, but it's her call and she "wants to use what she has" for now.....
 
Basically, she wants to capture melody/song ideas, into Home Studio. Her Casio WK225 puts out midi over usb. She's already doing that but wants to hear it's audio, so the I/O is what we want to implement for now. Getting both the midi and audio going is my wish and I think that will lead to more overdubbing .... though it may still be in the "sketches" realm.
 
She doodles on her very nice hand pan(this is her 3rd one and it's from Columbia), 2 Cordoba guitars (GK Pro Negra and GK Studio (cypress)), plus her Casio and some singing. All tracks will get "advanced/embellished" on "my side of the house" (see signature)! LOL    She might do some overdubbing on her rig, and has before in Audacity, prior to Home Studio. I'm all for replacing the usb mic or at least getting a better I/O and letting her use one the studio mics. Basically, she sees her set up as a sketch pad, to log ideas and sketches.
 
Thanks to all for all your responses... I am appreciative and impressed! Wow.
Michael
 
PS   As far as her soundcard, it is listed as a Realtek High Definition Audio, built into the AsRock Intel Z97 Anniversary mobo.

Actually, the onboard soundcard is the usb mic path. We haven't hooked up the Tascam yet and my questions were regarding "said path".
   Since the Tascam doesn't have a usb port for the mic, those two items won't be used together. The Samson mic is only usb.
 
My reason for coming here, is I have no experience/familiarity with Home Studio (which just came out), usb mics and all they entail. I am trying to set her up with a workable rig (I mentioned she has the same desktop i7 -4790k -4 Ghz Win 10 -32 gb as per my signature).  I would love to hear what she comes up, but it would probably involve making as easy and few steps as possible. She is predominantly busy with creating art and running the website for that. She doesn't want to have a set up like mine.
 
I appreciate your comments about the frustrations involved. I wondered whether I could get "her up and running" with what she has now. I understand what to do if/when we decide to upgrade the mic and the I/O.
 
Michael