Hi Paulisher,
I use a summing mixer all the time (mine is an SSL). Here's how I set mine up (this assumes your monitoring off a feed from the summing mixer):
1. route D/A outputs to the inputs on the summing mixer.
2. Route tracks directly to those outputs. Do not route ANYTHING to a master bus.
3. On your master bus set it's output to "none".
4. Take the summed "2 bus" output off your mixer and bring that into an input on your interface...for example, you might use the last pair on your interface, say 15/16.
5. Create an audio track in Sonar with an input of whatever you used for your input (in the above example I used 15/16). Label it "print track" or "mixdown" or some other descriptive title. Set its output to your master bus. This is the only track that outputs to your master bus.
Once that's done when you want to print your mixdown just hit the record arm on your "print track" and hit record. This records the output from your summing mixer to that track. The reason I set the "print track" output to the Master bus (which, again, does not have an output) is that it gives me a place to view the signal coming from the summing mixer aside from the actual recording track. For example, I like to have an analyzer and some other meters for the signal from my summing mixer so I put them in the Master bus fx bin and enable input echo on the "print track".
Another option for the master bus is to set it's output to a set of hardware outs that bypass your summing mixer to a separate input on your monitor controller. That way you can listen to the mixdown by selecting that input without the summing mixer in the chain (I frequently do this).
This also allows for easy versioning of mixdowns. If you want to create a new mixdown just clone your print track, delete the media on the new track, archive the old one and create a new mixdown. I use version numbers and dates on my print tracks so I can keep track of what is what.
Good luck
Dean
EDIT: Gah! AT beat me to it