I searched for this subject was surprised that I got zero hits, but here's what I am not understanding.
 
I have 2 bass tracks which I would lik to combine into one track. There is no overlap between the 2 I ended p this way while writing the tune.
Its a fusion instrumental and its still jut a proof of concept/demo
 
The reason I thought I should combine the tracks is that I want to check out the tune at a higher metronome setting to see if I should change tempos when I start over and thought it would be easier to reduce the number of tracks ,
 
I selelcted the 2 tracks,selected bounce from the track menu, selected  tracks as the items to be bounced and my 2 tracks were indeed highlighted.
I included all the options except 64 bit engine  and started the bounce.
But I got 2 tracks, not  2 tracks combined into 1.
 
Should I have bounced just one of the tracks into the other track?
 
THen I need to know how to have the tracks stretch with the tempo chance
 
 
 
 

Assuming your tracks are being output to 'Master Bus'.
Select the thus Master Bus as your source to bounce too.

HSC_III
I selelcted the 2 tracks,selected bounce from the track menu, selected  tracks as the items to be bounced and my 2 tracks were indeed highlighted.
I included all the options except 64 bit engine  and started the bounce.
But I got 2 tracks, not  2 tracks combined into 1.
 

 
And that's exactly what you got. Bounce is pretty straightforward, but the options are not entirely intuitive.
Once you realize what it's asking for it gets much easier.
For each source selected(highlighted), Sonar will create a new bounced track.
Obviously not what we want here.
To have Sonar bounce a combined track, the source must be a point where you can tap a combined track output.
This would be a bus or the sound card output.
The easiest way to do this is to select Entire Mix as the Source Category. This picks up your sound card output as the source(single).
Solo your two tracks, and bounce away.

Almost sounds like you bounced to "split mono"
 
There are lots of ways to bounce so it can be confusing. Here is my default easy method. Try these settings. I use this to bounce an entire song to a master trackso it should work the same for what you want. It should bounce what ever you have highlighted to on track. Output will be one track stereo.
 
Destination-New Track
Preset-ALL
Source Main Output- should be your main out on your audio device (mine is defaulted to my RME Babyface and grayed out)
Source Category-Entire Mix
Channel Format-Stereo
Mix Enables-all but audible bounce and live input

Is there a reason you did not simply drag the clips to the same track?

+1, hold the shift key while dragging and it will keep you from accidentally sliding the clip left/right.

Kalle Rantaaho
Is there a reason you did not simply drag the clips to the same track?

mettelus
+1, hold the shift key while dragging and it will keep you from accidentally sliding the clip left/right.

This.^^^^^^