This is the official announcement from Coinbase, there nothing to worry about that. Simply, If you hold BTC in Coinbase at the time these forks happen, they will credit the same amount of each coin. You get the new coin for free, just sell it once you get it.
But i think we should worry about the fact that they also did the same on august first regarding BCH. They made it seem like they'll credit their users asap but they did not. Instead they delayed it for months. Users can't still claim their BCH until now, coinbase said it'll be credit next year. So If you're planning to sell it don't ever store them on coinbase as its highly possible they will just hold it for a long time until the price falls.
My recollection is that they weren't deceptive about the Bitcoin Cash situation. To be fair, the BCH launch was super rushed, and the developers barely implemented replay protection in time for the launch. And I don't really think Coinbase acted as if they would credit users immediately.
This situation is different, though. The BTC1 repository hasn't even changed since August, and is unlikely to change at this point. And Coinbase officially said that they expect to have withdrawals operating smoothly for both coins "within a few days." I don't think they said anything like that with BCH. Coinbase wants B2X to succeed, so they won't make it difficult for users to withdraw it.