What you're doing is not the safe way recommended on this thread. You should have sent your BTC to a new address or addresses. If the BCH wallet you're using is compromised, it can steal all your coins with the way you have proceeded.
Now about sending BCH at the same time as BTC, it's not possible, so there is no risk of that happening. It's like asking if you can make a single wire transfer with dollars and rubles, not going to happen.
So the 1st post in this thread states to do things in this order:
1. Create a new wallet
2. Send all your BTC to this new wallet
3. On a separate computer, install a BCash wallet and import keys from your original,
now empty, BTC wallet
So that is pretty much what I did, only I did step 3 before moving my BTC to the new wallet. I'm now working on steps 1 and 2.
The reason I'm doing steps 1 and 2 is that I understand there would otherwise be a risk of loosing my BTC balance when I transact my BCC since they both share the same keys.
All 3 computers are on my local LAN with a gateway/firewall to the outside and all the wallets are encrypted.