If you wish to hold your bitcoins during the hard fork event, just withdraw them from the exchanges ( if you are using any)into a wallet like electrum and wait. No need to do anything else. You can choose your side whenever you like.
What you need to be careful is, you need to do it before the HF, or you will be at the exchanges mercy.
If then HF happens and it splits, will a wallet like Ledger Nano S give copies of both or does it have to be a software wallet?
Regardless of any wallet, software or hardware, exchange or own, until you have FULL CONTROL of your BTC wallet private key, any upcoming hard frock will create same balance on both chain, on current case both BTC and BTU.
Let me make it more clear, let's say, upto last block when hard frock happen, you have 5BTC on your wallet, as soon frock happen, you now have both BTC and BTU 5 units of coin on each side. As you control the private key of coin on both chain, ONLY you can spend, send, give, pay the amount on both chain.
Now come to the point where you don't have access to private key of your wallet, ie. balance on exchange or any 3rd party wallet provider, its up to the provider/exchange to make decision. Most of reputed exchange like Poloniex, Kraken, HitBTC already declared that they will give same amount of BTC to their user in case of hard frock. They will update your balance with same amount of BTU matching with your BTC balance on time of frock. As it is totally depends on your wallet provider to give you or not give you the balance. Batter you check directly with your wallet provider.
My suggestion will be to have your own wallet with private key known ONLY BY YOURSELF.