They say so in their FAQ:
https://support.coinbase.com/customer/portal/articles/2892985-segwit-2x-faqCoinbase will support buys, sells, sends, and receives for both BTC and B2X. Services for both chains may be disabled several hours before and 24-48 hours after the fork. We’ve posted a general timeline here, and plan on sharing more specific details as we get closer to the fork.