Ball Sit Ups
In the video above. First two reps are as you would do them by yourself. Second two reps are done if you had a partner.
To do with a partner:
Lock legs using feet and ankles. Fitter persons legs go on the outside.
Pass ball at the top of the movement.
Either rest at the top of the movement while your partner does their rep OR do an extra rep without the ball as they are doing their rep.