Join forces for an engaging abdominal workout that adds excitement and inspiration to your training regimen. Begin with seated partner twists, in which you sit back-to-back and pass a medicine ball to each other, working your core with each revolution. Try plank high-fives, in which one partner holds a plank and the other reaches over for a high-five, which tests your stability. Incorporate partner leg lifts, in which one person holds the other's legs as they elevate and lower them to…