I can only guess what you're trying to do from your sketchy description. You want to track the positions of two persons by some means, then use their positions to project videos on their t-shirts so that the persons may move but the videos always follow them. Do you want to do invisibility like some Japanese researchers do or just art? If you project what's behind the persons onto their clothing, you get some invisibility effect.

Well, here is my simple thought: have one person wear a red led button on his shirt. Use a camera to capture their motion and in each frame find where the red led is. Do the same for the other person with a blue led button and track his position. Then use the results to construct video contents that will cover the two persons at the correct location. But unless you can track their distance from you, you will have to confine them within a fixed distance from your projector. Hope this helps.

Hi

the Wii has an excellent little infrared camera. So if you put infrared reflectors on the people, you could use a powerfull infrared spot, with the right filters (important), that would be invisible to the audience but bright enough for the wii to track precisely. (just speculating on an idea Andres Leon had during a workshop, I've never actually seen this in action)

Alvaro