In the case of two chains, Bitfinex will list the Segwit2X fork as 'B2X', and the real bitcoin as 'BTC'.

You can read it on their new T+Cs about the segwit2x CSTs
1.1“BT1” means a CST representing the Incumbent Bitcoin Blockchain;
1.2“BT2” means a CST representing Segwit2x consensus protocol;
1.3“B2X” means a Digital Token converted from BT2s at or after Contract Settlement, as set out in these T&Cs;
1.8“Incumbent Blockchain” means the currently existing Bitcoin blockchain that is exclusively used based on the consensus protocol as set out from time to time in the repository https://github.com/bitcoin/bitcoin
;
1.9“Segwit2x” means the anticipated Bitcoin blockchain that will be exclusively used based on the consensus protocol as set out from time to time in the repository https://github.com/btc1/bitcoin
;
3.Settlement—Generally & BT1s: At Contract Settlement, CSTs shall be converted on a 1:1 basis to Digital Tokens on their respective blockchains at Contract Settlement. In the case of BT1, the blockchain shall be defined by the consensus rules used on the Incumbent Blockchain, currently designated with the ticker symbol BTC. If no Incumbent Blockchain exists pursuant to these T+Cs, BT1 tokens shall be deemed to have a value equal to zero and shall be removed from the platform. All open orders for any BT1 trading pair will be canceled at Contract Settlement.
4.Settlement—BT2s: In the case of BT2, Segwit2x shall be deemed to exist only if a blockchain has diverged incompatibly from the Incumbent Blockchain. Any settlements of BT2 shall be to B2X. If no Segwit2x blockchain exists pursuant to these T+Cs, BT2 tokens shall be deemed to have a value equal to zero and shall be removed from the platform. All open orders for any BT2 trading pair will be converted into open orders for corresponding B2X trading pairs to ensure pricing continuity and minimize disruption for users trading BTC pairs on leverage prior to the Chain Split Event.
link: https://www.bitfinex.com/legal/cst/segwit2x