This is what will happen.
On the day of the fork at block 494784 these are the possible scenarios.
1) All the signatories to the NYA agreement have upgraded to btc1 and Segwit2X becomes BTC.
2) The signatories to the NYA agreement do not upgrade, in which case Segwit1X remains BTC.
3) Some of the signatories upgrade, and this is the most likely scenario. We won't know which is the preferred coin until after the first block is mined and confirmed. We will then see the Chain Death Spiral come into play. None of these two chains have Emergency Difficulty Adjustment protection and so one will come to a grinding halt.
If it is Segwit1X it will most likely be forked off with a different proof of work and replay protection. It will have to undergo the same process that Bitcoin Cash went through. Segwit1X coin will have to be submitted to exchanges to determine its' value. Coin splitting will come after. Not before it has proven itself to be a viable and valuable coin.
If it is Segwit2X, then it will most probably be abandoned.
Exchanges may stop all deposits and withdrawal until this is resolved. Thereafter it is business as usual.
4) While all this is happening Bitcoin Cash is still in the mix complicating matters. It could conceivably be the most profitable coin to mine and starve both chain off mining hashrate. A protracted and drawn out tussle between Segwit1X and Segwit2X may lead to greater use of Bitcoin Cash for bitcoin transactions. More likely than not this will be resolved very quickly.
5) What of Bitcoin Gold? Nothing. It will have to go through the same process. Submit their token to exchanges to determine value.
6) Recently the price of bitcoin have risen sharply and altcoins have taken a beating. This is due to people selling altcoins and buying into bitcoin hoping to own 2 coins after the split. Many believe that the two bitcoins will have a higher valuation than before the split.
They will be disappointed. After the hard fork, the only difference between the 2 coins is 1MB and 2MB. If 1MB wins nothing changes and there won't be a second coin. If 2MB wins 1MB must fork off with a different proof of work, and the only reason to support such a chain is to support the developers. It is unclear if the Core developers will even be interested to work on this chain. Others may come forward to volunteer their efforts but it is unlikely to make any headway without a clear point of difference and a roadmap.
7) If 2MB wins and the whole Core team rage quit, will Segwit be relevant? Who will continue development? If segwit becomes irrelevant what becomes of Segwit2X? Of course the best outcome is for the Core team to swallow their pride and move to 2MB. However this won't happen because it was never about bitcoin or the community. It was about control of the community through Bitcoin.
Source - bitcoinandtheblockchain.blogspot.com
Source - dharian.com