Can I do this?
 

We play small to average size venues, and my Traynor 200 does the job fine, however the one big 1x15 has a fat but simple tone. I just picked up a Behrenger 120 Ultra. The aluminum cone is a completely different bright tone.

What I am wondering is, can I use the Behrenger as an extension cab, thereby getting both a fat and bright tone? I realize I would need to keep the volume down to keep from overpowering the 120, but I never use more than 50% on the Traynor anyway. The Behrenger has a jack on the back labeled "amp in".

Are these both combo amps? What is the impedance of the spkrs in each? What is the minimum impedance of both the amps?

The trick might work, but you really need to check the owners manuals for both amps to see what they say about impedence (ohms) and power handling (watts) for those external speaker/external amp jacks. Another option that would probably be safer and might even sound better would be if one of the amps has a pre-amp/effects output that you can cable to the other amp's pre-amp/effects input. In that case, you'd have both power amps driving the speakers in the way for which they were designed, and you'd get the total bi-amped power. I've done this in the past, running my EBS Drome's post-eq line out to another amp, and it worked amazingly well as "poor man's bi-amping".

Dont plug your speaker out into the amp in jack!! It is likely in front of the power stage. If your traynor has a pre amp out or effects send, that could be plugged into the amp in jack and would use the pre section from the traynor to drive the behringer power amp and speaker.

Dude, I just glanced at the on-line manuals for those amps and whatever you do, DON'T connect the Extension Speaker Cabinet output from the Traynor to the "Amp In" of the Behringer! As I read the Behringer manual, that's actually a PRE-AMP in, and running expansion cabinet signal through it could very possibly do immediate and permanent damage. It LOOKS like you should, however, be able to patch the FX Out of the Traynor to that Amp In, and have the bi-amping configuration I was talking about.

You could probably take the "EFX Send" or "Line Out" of the Traynor into the "Amp In" of the B-word.