Magic Jar says
You can attempt to possess any humanoid within 100 feet of you that you can see … The target must make a Charisma saving throw. On a failure, your soul moves into the target's body, and the target's soul becomes trapped in the container.
So could a character with a Magic Jar use it to trade control of their body back and forth with another willing soul? Even if you can't relent and choose to fail the CHA save, it would be successful at least some of the time.
Best Answer
Sure
Magic Jar provides opportunities for you to enter or leave a creature's body.
If you wanted to do this quickly and often, you might be frustrated by the fact that the creature would be unavailable for 24 hours if it ever succeeded on the saving throw. And if the creature ever became unwilling, it would probably run away from the container once it had control of its body back. But otherwise, go ahead.
But watch yourself... literally
Note that Magic Jar lasts "until dispelled." And if the spell ends when you are more than 100 feet from your original body (or your original body is dead) you die. As such, if you are "sharing" another creatures body far away from your original one, and someone casts "dispel magic" upon the magical effect which is permitting your soul to occupy that body, then the Magic Jar spell could end. In that case, you would instantly die. In order to do this safely, you'd need to keep your body close by, and would also need to periodically feed it and keep it in relatively good health.
So take care with this strategy. As much fun as it could be to do an "All Of Me" style zany romp sharing one body back and forth, keep in mind you're messing with powerful necromantic forces here. If you're not careful, it could backfire with severe consequences.