The only time I did two Firm workouts a day was when I was doing the Firm Express rotation. I would never attempt to work out longer than 70 minutes a day, including warm up, cool down, and thorough stretch, unless it was a relaxing yoga afterwards. I don't think beating yourself up twice a day every day (which sounds like what you're doing, if you feel like you're too tired or too sick or not recovered) is the way to get to your results faster. Sounds like a way to get set back
. I know some people around here can do it with great results, but if that's not how your body works, don't risk it.
On the other hand, maybe you are not able to do it because you're not fueling properly before or after your workouts? I have no experience in this area, but I know Jeanne Marie has no problem pounding out multiple intense workouts a day. Maybe she will chime in.