If you add another line into your account, you are still the owners of both lines. There is only one combined bill for 2 lines. You can separate the amount yourself.

However, payment is another issue. There is one login for 2 lines. If you want you can share that to the other person so he/she can use a different credit card to pay,but  that person would be able to see all of your call history,data usage,etc. 

Since you are adding a line, both lines will still show up on one account and therefore one bill. However, you can pay separately by manually inputting the amount you owe for each line and use different credit cards to pay it off on your Self-Serve account. 

If you want to keep these two lines in separate accounts, you will need to get a transfer of ownership. Note there will be a $50 fee for this service. https://www.koodomobile.com/en/help/transfer-of-ownership-town