I just want to record audio like I'm using an old 8 track recorder ...

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The big thing here in what you say is you'll be using a drum machine. There's a few ways to make a huge improvement over the typical sounds those make. It might be worth your time to learn just a little bit of midi so you can create a very realistic drum track using Sonar and midi. 
 
You can use the drum machine sounds as well and with a little setting up you can sync that to Sonar so Sonars tempo drives the machine using midi sync. I have mine set up that way. I use it just to get the song rolling and then replace that with a midi drum track latter. And you mentioned keyboards so that's another reason to use midi. 
I made a simple tutorial on using Midi in Sonar if you want to kick the tires. 
http://www.cactusmusic.ca/Sonarmidi.htm
 
 
Typically I make a bed track of all the music and then record the vocals. 
The bed track can be audio or midi or both. 
The headphone mix is as simple as setting the direct record input level in balance with the output of your bed track playback. 
On your interface the output is called  Input- Computer. Just adjust this to achieve the correct balance. 
 
 
 

The old Alesis machines do still have their uses and contain quite a few good sounds. I still use an SR16 I bought in the 90s sometimes for percussion sounds. Usually as a MIDI controlled module though, rather than attempting to program it via it’s own pretty horrible interface.

For synth-style drums I use a MachineDrum or an older model MFB which has some very solid kick sounds in it.

I wouldn’t attempt to make a track that sounded like a human played it on a kit on any of those though, that’s a job for customisable algorithmic tools like Logic’s Drummer instrument or Jamstix as far as I’m concerned. Then take what comes out of them and edit the MIDI as required. Far, far faster and easier than spendign hours programming an entire “realistic” 10 minute drum part from scratch.

As I primarily record "traditional" (old style) country, the Alesis SR-16 "31A" program is my go to for many.  I use others but that is my most used.  The SR-18 has built in 3/4 (Waltz) program that they forgot to include in the SR-16 factory programs.  I have a 3/4 user program in my SR-16 that the salesman at a local music store programmed for me.

I used to use an Alesis SR 16 myself but I've found there are so many other, better sounds out there (more realistic sounding drums) set up as VST-I's.
Also going to be a little difficult using multiple headphones with the Tascam 2X2 unless you have an external headphone amp.
 

Cactus Music
Actually Chuck he didn't say multiple, but you more or less need 2 for recording vocals, one for the "Engineer" and one for the "talent",, I used to get away with headphone splitters for this. Radio Shack. 

 Did I miss something ? He said "My wife and I"...
That sure sounds like multiple to me.

MDShinn
 
My wife and I just want to make great recordings of originals for ourselves.

 
And this...
 

MDShinn
 don't know how to setup good headphone levels that result in good recording levels 

 
Doesn't sound like a Radio shack splitter is the best remedy.
 

I said,, I used to get by not that it was ideal but it works perfectly fine in a lot of situations. 
I myself have a dedicated mixer just for headphones. 
We sort of need to hear more details I guess. 
Like, are you planing on recording songs "live" in one take, or overdubbing each part?