At the time of the fork, your previous key now gives you access to two independent wallets one BTC, one BCH. They both start with the amount you had before the fork. There are no "pre-fork" nor "post-fork" bitcoin, there are now only one key opening two different wallets.
The two wallets are not compatible anymore, it means that you cannot move some coins from the BCH wallet to BTC wallet, nor BTC to BCH.
If you send BTC to your BTC wallet, then your BTC wallet will get the coins, and the BCH wallet will stay unchanged.
If you want to fill the BCH wallet also, you have to make another transaction, on the BCH network, of the same amount.
Best pratice would be to separate your BCH wallets from BTC wallets. You can do this with Electron Cash software with the "recovery passphrase", as described here: [https://thebot.net/threads/tutorial-how-to-get-your-private-key-from-blockchain-info-redeem-your-bch-from-blockchain-info.396919/]. Don't forget that until you move the coins to new wallets, the same key will give access to both BTC and BCH wallets, even if they have different lives, so keep them safe!