Jef09,

I am a Beachbody Coach and I have finished Insanity, repeating month 2 twice. I also have completed P90X. I did both while cycling this summer.

I think that they work well together. Insanity will definately help your cardio performance, while giving you minor strength gains in your upper body (this surprised me with no weights in the work outs - but there are TONS of push ups, V push-ups, etc.)

As long as you listen to your body, and make sure you are giving it recovery time, you should be fine.