I have a small subwoofer that I would like to use on our home studio. Is there a way in Sonar (such as using EQ) to create a virtual crossover so that recorded or virtual instrument audio can be split into two signals, with the signal above a specific frequency sent to one output (to small speakers) and the audio below the specific frequency sent to the subwoofer via a second audio output?
 
 

Assuming you have enough audio outputs to support the Sub on an output by itself than yes you can do this fairly easily.
 
Just create 2 additional busses on your project and add a send for one of the new busses in your existing Main output bus. Then direct your main output to the other new buss.
 
Add eq to each and create the crossover curve you want to use. For instance you could set a hipass on one and direct that to your main speakers and a lowpass on the other and direct to your Subwoofer.
 
This is off the top of my head and is over simplified but should get you on the right track.
 

gracias!

If you do this, watch out for a bump or dip in the overall eq at the crossover point and volume balance issues between the sub and everything else.
 
Having some form of room analyser so you can put white noise through the system and check the response is flat comes in very useful.