Dual Plank
This exercise is extremely difficult with regards to balance, and as such should only be tried by confident persons.
Place your feet either side of the center on the rear ball, and rest either your forearms or palms on the front ball, as in a standard plank position.
Focus on keeping your core stability muscles contracted throughout the movement to prevent your hips sloping down.
Stay fixed in this position for a long as comfortable, without feeling any discomfort or poor technique coming into play.
A variety of hand positions can be utilized to increase the difficulty level, likewise you may want to try this exercise whilst performing some slow press-ups, (very difficult).
Begin the exercise with larger Swiss Balls of the same size and inflation, progressing to different size balls if you desire.