Could muscarine be an antidote to atropine poisoning? According to Wikipedia,


  "Muscarine poisoning is characterized by miosis, blurred vision, increased salivation, excessive sweating, lacrimation, bronchial secretions, bronchoconstriction, bradycardia, abdominal cramping, increased gastric acid secretion, diarrhea, and polyuria. If muscarine reaches the brain it can cause tremor, convulsions, and hypothermia. Cardiac ventricles contain muscarinic receptors that mediate a decrease in the force of contractions leading to a lower blood pressure. If muscarine is administered intravenously, muscarine can trigger acute circulatory failure with cardiac arrest... The specific antidote is atropine." 


However, the wikipedia article on atropine also states,


  "In overdoses, atropine is poisonous."


Could muscarine be used to treat atropine poisonings, just like atropine is used to treat muscarine poisonings?
In short: no.

It is not that simple. According to Wikipedia, atropine works as a muscarinic antagonist, which blocks the action of acetylcholine at the muscarinic acetylcholine receptor. So, the mode of action as an antidote is to prevent muscarine from binding to the receptor, thus inhibiting its effects on the body.

The poisonous effects of atropine cannot be reversed by muscarine, though. That may be due to the fact, that it is able to displace muscarine and not the other way around.