The triaxis is so smart!! I use it to control everything. First off if they sound a little whack by themselves, I think they are out of phase. Radial said the most common position would be at 0 instead of 180, on your switch. I use lift and leave the other at zero. Let me take a sec to explain my set up, this is from another topic I posted about how the pod pro can be used too, (you can use this info for any midi effects unit).
Here are some ideas. I actually use my pod pro to control everything with the floor board with one step. My Pod pro, Triaxis and Dual Rectifier 2 channel head. By hooking a midi cable from the pod pro to the Triaxis and plugging a 1/4 inch cable from the dual rectifier channel switch plug on the back, to 1 of the 4 switch jacks on the Triaxis. Now I can choose to activate the effects loop if I want to hear the Pod and it's effects, which is only 10 percent of the time for the delay swell effect. The Pod pro is in the Triaxis' effects loop, and I have the loop turned off most of the time for pure MARK sounds. The Pod pro will always switch all units even if I don't have the loop activated on Triaxis. Makes set up quick, only one midi floor board!!
With this set up you can do anything!!
Use the tuner on the pod,
switch all 3 units with one step on line 6 floorboard or any other midi pedal.
Mix/blend Mark IIc+ or MarkI-IV sounds with Dual Rectifier sounds at the same time( with the Switchbone a/b/y switch and 2 4x12 cabs or sometimes split one 4x12 cab)
Or clean and distortion at the same time,
Change channels on the Dual Rectifier with each patch on the Triaxis
I also have an Ernie ball volume pedal in the DR effects loop, to control the mix of the Dual rectifier and the Cabinet it is going to, with the other cab.
With this I get all the great tones with no effects most of the time, unless I want them. To bypass the Pod Pro's amp models (not the pod xt) and use only it's effects for a pure sound, make a patch with the TUBE PREAMP that is like a bypass, turn everything to 12 o clock. They suggest this in the Pod manual. Hope this helps, I have been really excited about how smart my equipment is and I don't need to buy any extra stuff like the Vodoo Lab Ground Control units. This saved me alot of money and set up time!!
So to answer your question I can plug the dual rectifier channel jack to the triaxis. Having the switch activated in the patch changes the rectifier to red, having it not activated, puts the dual rectifier on yellow. And my line6 floorboard controls my triaxis, which controls my dual rectifier. Let me know if you have any questions. Also FYI I put the radial in my rack and leave both amps activated all show. I just use my volume pedal to remove one amp or fade in, or silently tune.