Could muscarine be an antidote to atropine poisoning?
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.