I double up the cardios all the time. I often do CTX All Step followed immediately by POwer Circuit. Another fave combo is 10 10 10 followed by Kickbox, in those orders.
I don't do the muscle conditioning which follows. I need upper body strength, not endurance, so if I double up with the cardios I will do PUB on another day, often after a single CTX cardio.
Results?
Hard to say because I don't follow strict rotations, just my heart and low boredom threshold. But, I will say that it breaks a cardio threshold. It really jacks up my cardio endurance. I really experience feeling FIT on days when I do this. And the other result is that I beam like a Cheshire cat when I am done.
I double up CTX cardios once a week only. I imagine, it is akin to MIC or BOdy Max in the results it would produce.
CLare