I currently have a cheap M-Audio Oxygen 8 which is only 2 octaves wide.  Good for browsing samples, but limited for playing.  I'm hoping to acquire a keyboard with a lot more octaves so I can play two-handed and use key switches.  I've had a regular wood piano all my life and I'm used to it.
 
Instead of buying an 88 key controller, I've been thinking of a 61 key controller (like the A800Pro) which would permit two-handed playing, but might be a bit tight for also key switching.  Can I use my Oxygen 8 for control keys and the 61 key controller for playing ?
 
I have a feeling doing so might also make a bit more sense ergonomically and functionally, or do you have to play the switches and left hand at the same time and having them close together is an advantage ?  Or does using key switches for expression limit you to playing the music with the right-hand only ?
 

Do not know but I use my Keystation Pro 88 and a Peavey 16X controller together.  I do not think you can overlay them never tried.

 
Seems to me if both keyboards are sending on the same midi channel they could be seen by Sonar as if they were a single instrument ?
 
I'm really only thinking about the piano keys, not controller buttons and switches.
 
Mostly about using my Oxygen 8 for key switches though I'm sure I'll think of more uses for it.

Paul P
 
Seems to me if both keyboards are sending on the same midi channel they could be seen by Sonar as if they were a single instrument ?
 

Yes, it's all a matter of how you set it up in Sonar. I use up to 3 keyboards in the creative workflow and jump between sounds by changing the MIDI channels, sometimes 2 of them end up being assigned to the same channel, so I could use either one ... haven't seen any negative impact from that yet ...

 
Thanks FreeFryBertl, I just wanted to make sure.
 
Now if the canadian $ would just go back up.  The A800Pro is suddenly 100$ more than it was a month or two ago.
 

Paul P
 
Now if the canadian $ would just go back up.  The A800Pro is suddenly 100$ more than it was a month or two ago.
 

yeah these currency inflicted price changes are nuts ... the Stingray 5 I bought last year now goes for additional 1000 € (!!!) ... I'm glad I invested in that one last year ... of course, I did not need it, but now I would not want to miss it :-)

 I don't think you can combine two midi in devices to record the same channel at the same time.   

Even if the controllers are set to the same channel, Windows and Sonar will see them as two different devices because each will set up and communicate through its own MIDI ports.

One work-around is to point two MIDI tracks at the same synth. MIDI track 1 handles one MIDI controller's data and MIDI track 2 handles the other controller. If both controllers and tracks are set to the same channel you should then be able to use one controller as keyboard and the other as a source of CC data. They may even work as two keyboards/octave splits, but in that case the MIDI notes will be spread over two tracks.

Using two controllers for the same synth generally means you need both tracks open in the piano roll etc. at the same time to edit the data.

What TLW said, Each midi track can only select one midi input source at a time. 
If you were using real midi ( DIN) ports then you would need a midi merge box like a Yamaha  ymm2.  
I think the 2 midi track trick might work but I would think only the focused track with the input echo lit would be heard. 
 
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Interesting....
 
But what about midi Omni... Where any device transmitting a channel would be picked up. For example I have a pad controller, Edrums and midi keyboard all set to trigger and control drums.. Would that not be an option?