What did the Buddha said and taught about sharing demerit?

The answer to this question can only be that there's officially no such thing as sharing of demerit.

Despite that, the closest I can offer is telling others of one's past misdeeds as an example of what they shouldn't emulate, and also as an example of how it is possible to change to become better.

For e.g. the Angulimala Sutta shows how one shouldn't be a violent person, and yet it also shows how a violent person can change to become an arahant.

If we can agree that "demerit" is "apuññaṁ", then the Buddha tells us:

AN2.134:1.1: “Dvīhi, bhikkhave, dhammehi samannāgato bālo abyatto asappuriso khataṁ upahataṁ attānaṁ pariharati, sāvajjo ca hoti sānuvajjo ca viññūnaṁ, bahuñca apuññaṁ pasavati.
AN2.134:1.1: “When a foolish, incompetent bad person has two qualities they keep themselves broken and damaged. They deserve to be blamed and criticized by sensible people, and they make much bad karma.

Sharing such demerit could then be understood as:

AN2.134:1.3: Without examining or scrutinizing, they praise those deserving of criticism
AN2.134:1.4: and they criticize those deserving of praise.

The Buddha continues and says this about merit:

AN2.134:2.3: After examining and scrutinizing, they criticize those deserving of criticism
AN2.134:2.4: and they praise those deserving of praise.
AN2.134:2.5: When an astute, competent good person has these two qualities they keep themselves healthy and whole. They don’t deserve to be blamed and criticized by sensible people, and they make much merit.”

However, until one is free of delusion, it may perhaps be best to pay proper attention and say little until wisdom is gained. For example, praising organized crime bosses for their local generosity may also support their ongoing criminal ventures. And it's usually fatal to criticize a crime boss.

MN8:16.1: Truly, Cunda, if you’re sinking down in the mud you can’t pull out someone else who is also sinking down in the mud.
MN8:16.2: But if you’re not sinking down in the mud you can pull out someone else who is sinking down in the mud.

It's actually quite hard to escape demerit by focusing on demerit. It's much easier to escape demerit by focusing on merit. That's why it is critical to practice and follow Buddhist ethics.

MN8:12.7: ‘Others will speak divisively, but here we will not speak divisively.’
MN8:12.8: ‘Others will speak harshly, but here we will not speak harshly.’
MN8:12.9: ‘Others will talk nonsense, but here we will not talk nonsense.’
(...omitted for brevity...)

(submitted by Anonymous)

"Now I have to think whether sharing the hunts of Robin Hoods would fall under share of merits or demerits to make the circle of escape closed within" --Anonymous

Exactly. From a Buddhist point of view, stealing from the rich is still stealing.

Encouraging others to demeritious deeds (a-ku-sala) is one of the four ways of evil kamma (aside of bodily doing, by approve, rejoicing, by speech). Praising what deserves dispraise, leads as well toward low realms hell. Like the sharing of merits (kusala), telling of good conducts and let others rejoice with it, share of demerits has same effects like sharing merits, for one who shares, and those who join by approve and delight but into downwardly direction with painful results. Those who share disaprove of what deserves praise, like tjose who join this demerits head as well toward hell.

Like a single candle can light countless others, such is the possibility of offering a share of A-Dhamma and can be used to burn off much. Everybody knowns the phenomena of demerits, known as root of wars and disasters and everybody has seen samples of joining demerits.

Medias are full of shares of demerits and looking at upvotes and likes shows as well how strong and rich the part-taking on demerits arises. People still wonder why humans, at large, meet low realms after break up of the body... Yet who knows of places where share of merits is custom and finds approve? Most of you might not even have ever met such places, yet so deep in the forest that trees all around are not seen, in regard of share and rejoice with demerits... just look at this occasion right here... "buddhism exchange, they call it..." Sad but true.

As today tools of shares are much faster, one is wise to better do not spread of what one isn't sure whether right or wrong, good or bad.

Blindness and lack of discerment aren't a protection of results, note that not-knowing, not-understanding is the root of suffering and who would do anything wrong if knowing, at least.

Many might think that things are easier this days, but actually much more dangerous for not so gifted and discerment and good association-lacking common people.

And in regard of an Unfettered Heart: Such has gone beyond merits and demerits by own deeds, beyond accumulation, yet, out of compassion, still shares samples of both to teach it's distinction, teaches benefits (for long term happines) of merits, teaches loose by demerits, and both and in cases all does not work out, he 'kills' ones in regard of getting approached and shares further on.

May merits and demerits, right discerned, been always seen by those who ever little capable, forwarded and take part or not on them.

Take care and as a khema proverb tells: turn your tongue better 100 times aronnd before speaking.