First of all, make sure your Ledger Nano S is updated to the latest firmware for splitting coins.
Second, during the fork event, do not make any transaction on the network and wait a few days because of the missing replay-protection.
After the fork, you have to split your coins in the Ledger Chrome application or on a new transaction, it will not just execute it on the standard Bitcoin chain but also on the Segwitx2 chain that means you lose the coins on Segwitx2 chain.
However, you get a warning and an invitation for the split before doing a transaction.
More information will be posted on the
Ledger Blog.