I would wait until you're done with Insanity. You burn crazy amounts right now, and you need a surplus of calories to build muscles. You could, conceivably, add light weights in now as long as you're supplementing with tons of protein. You need to give your body time to recover, and two really intense work outs a day may cause you to seriously fatigue.

I have done the hybrid programs and this seems to work out well. BeachBody has worksheets for P90X / Insanity programs. I feel like you get the best of both programs with the hybrids.
good luck

I did a P90x/Insanity hybrid program for 3 months. Basically I substituted all the P90x cardio workouts for Insanity ones, since even P90x plyometrics doesn't hold a candle to the easiest Insanity workout, in my opinion. My results were great. I didn't lose much weight (I don't have much to lose), but I made some pretty noticeable gains in muscle; of course, that is mostly attributed to the resistance in P90x, so it remains to be seen if substituting cardio for Insanity helped me out any. In my mind, it definitely did because I burn more calories doing Insanity than any cardio workout from P90x.

The schedule I followed is here: http://pledgetostayfit.com/wp-content/uploads/2009/08/hybrid-schedule1.png