I'm checking How does the Lightning network work in simple terms? answer example:
1) Putting funds into Lightning We both send 0.5 BTC into a shared 2-of-2 multisig address. This requires a transaction on the Bitcoin blockchain. As a "balance sheet", we each create a 2-of-2 multisig transaction that pays out 0.5 BTC to you and 0.5 BTC to me. I sign one payout transaction and give it to you. If you want to put it in effect, you need only add your own signature and broadcast it to the Bitcoin network. Vice versa, you give me your copy of the payout transaction that you have signed and I have not yet. The "balance sheet" is a regular 2-of-2 multisig transaction in the Bitcoin network, it just isn't valid yet, because it doesn't have all signatures.
So in the first step you make a regular transaction from your address A to our common 2-2 address C, and I make a regular transaction from my address B to our common 2-2 address C?
I have one key of this 2-2 multisig address C and you have another?
Address C now has 1BTC.
Now from this address C I return 0.5 of its 1 BTC to your address A and sign with my key, and you return remaining 0.5 to my address B and sign with your key?