https://blog.coinbase.com/bitcoin-segwit2x-update-b69a1c1e5eceScenario 1: network is unusable
If support for the fork remains at current levels, or decreases, the Bitcoin2x network will be unusable. Coinbase will not support withdrawals or trading as it will not be possible to move these assets. Currently we believe this is the most likely scenario.
If the network gains support at a later date, we will enable Bitcoin2x withdrawals from the platform.
Scenario 2: network is usable
If transactions are being confirmed at a reasonable speed, and miner support is strong, we will allow Coinbase customers to withdraw Bitcoin2x. We will not immediately enable buys and sells as previously stated, but we may enable them at a later date.