One position I've already described, is with the ass-person crouching down to receive his friend from behind. The advantages are easy and fairly deep entry and freedom of movement. Those of the penis- person include thrusting, grinding (rotating the hips circularly as you'd do to play with a hoola-hoop), rocking (turning the pelvis from left to right), and body motions like swaying from the knees. These motions allow different experiences for both partners. Those of the ass-person include complementary thrusting (the same as for the penis-person, except that your timing and his are slightly off), counter-thrusting (where you push as he pulls), grinding or rocking as he thrusts, and body movements. All these motions are easy to learn, once you feel save enough to try them and someone can help you a little.

The disadvantages of this position are both people having to hold themselves up (getting tired and not having free hands), the penis not pressing too much against the prostate, and not being face to face (if that's important to you).

A variation on this position has the ass-person lying flat on his stomach (with his hips raised a little on a pillow if you like). This allows him to relax more and he has his hands free to do whatever. The Penis-person lies on top and thus also can be more relaxed and have free hands. However, penetration is not as easy and you can't thrust as deeply. Also, the ass-person is weighed down and restricted in how he can move, through the penis-person can still more freely.