Then they will do a genuine hard fork and two coins. But in order to REALLY do so, that means, when they will start producing *incompatible blocks* and mine on one another, they have to realize that they will make an altcoin, losing bitcoin's first mover value proposition, which is essentially the only thing that keeps it number one for the moment. So I wonder whether they will really do that. In any case, if they do, there will be two bitcoin chains, two coins.
ETH/ETC all over.
Indeed. It will be a genuine hard fork / split, which would split off BU into their own altcoin. Something nice I found on reddit:ETH/ETC all over.
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Consider this thought experiment: you create your own altcoin, remove the 21M coin limit and make yourself the central issuer. You are the only user of your coin, but you manage to build/buy enough miners to put in more proof of work into your chain than the total work put into the Bitcoin chain. No one else uses your coin. Is your coin now Bitcoin? You can even use the same genesis block made by Satoshi. I hope this example makes it clear that your coin is not Bitcoin, because everybody else is using the Bitcoin they have always been running, even if is backed my less mining power than your own coin.
Similarly, you somehow attain 51% hashrate and split at one point (changing some rules along the way). Is your coin now Bitcoin? Whoever answers this question with 'yes' should visit a psychiatrist. 
I think it would be a "good thing", because then the market can choose. But I wonder whether they will dare to do so.
Bitcoin's first mover advantage is still too valuable in my opinion. It would be great if bitcoin could hard fork into two coins. But I don't think it will.
Agreed. Well, now we know who is actively trying to harm that.Bitcoin's first mover advantage is still too valuable in my opinion. It would be great if bitcoin could hard fork into two coins. But I don't think it will.