Hi Lorrie,
I believe I am on the same day as you for the X. I did CardioX this am before Chest, Shoulders & Tris. It doesn't have much momentum, but the intensity will really be up to you I think. It is a nice little mix and not real challenging, but for this program I think it is appropriate.
I am going to use it as the plan suggests to begin with (for Doubles, but I am doing both workouts back to back) just to see how it goes. Some days are Cardio and PlyoX together...so I don't think I will reach for a Cathe on those days.
With that said, I think you could sub out a Cathe from time to time..I added some because I was having withdrawls, and am seeing results already from Tony anyway. But I am trying to be careful about just how much intensity I use, because I really do want to see some strength and mass gains. I have already decided that after the 90, I am going to do a rotation of Cathe's to cut fat.
Hope you are enjoying P90X...
Julie