Okay, a married couple I know share toothbrushes. And their son uses the same toothbrush. Their excuse is, since they share so many... bacteria and fluids (cough) anyway, they're not doing each other harm by sharing a toothbrush.
Other than the "ick" factor, I think it's a very bad idea.
Other than the "ick" factor, I think it's a very bad idea.