CrossFit works the body through various modalities - weightlifting, metcon, and gymnastics. The gymnastics-based movements include all bodyweight exercises, like pullups, pushups, situps, squats, handstands, burpees, and the like. The monostructural metabolic conditioning, also referred to as metcon or cardio, comes in the form of running, rowing, swimming, biking, and, of course, jumping rope. There are many ways to combine metcon and gymnastics work into a powerful CrossFit WOD to give a potent one-two punch to your training.
One of the CrossFit benchmark workouts that combines situps and double under rope jumps is "Annie". This involves 50-40-30-20-10 rep rounds of double unders and situps. We've seen times in our gym range anywhere from 7 minutes to 30 minutes depending upon one's mastery of the double under.
If you need to work on your double under, buy a jump rope and practice, practice, practice! Then do "Annie" and see how good you can be.
Lora and Linda hammering away at "Annie". Notice Lora's double-under form - upright posture and only jumping high enough for the rope to get under the feet. Jumping higher than you need is a sure way to burn out on double unders.