When my wife was on bed rest when she was pregnant with our daughter I asked if it precluded sex. The rhetorical question I got back was:
Does it get her heart rate up?
The answer is, yes it is a form of cardio workout. The question then becomes a matter of the quality of cardio. In other words, how high does your heart rate go? You want to be in the aerobic zone to do what your plan wants for you. You don't want the heart rate too high, nor do you want it too low.
Of course the other side of the problem is a willing partner for 2x/day. You or your partner may become sore and need a break--particularly if you are doing what is necessary to keep your heart rate up. Using a heart rate monitor will help you know how your cardio work is doing, but it lessens the enjoyment of sex when you are concentrating on something else (like your heart rate).
Besides, do you really want something that is meant to be fun to now be viewed as work? I'm of the mindset to keep work and play separate. While merging them makes work more fun, it makes play less enjoyable. Keeping them separate activities works better for me.