Hi there,
So you will have to do something known as a transfer of ownership (TOWN) of the line that is not the account owner. More specifics on this can be found in the link below.
Just keep in mind this will incur a $50 transfer fee to the person who is leaving. When you speak to a rep via a call back, make sure you're both available to speak to them.
Doing a TOWN keeps everything the same for you in regards to your numbers and plans, they just seperate you onto two seperate accounts with seperate bills. If you have more questions, let me know. 🙂👍
https://www.koodomobile.com/en/help/transfer-of-ownership-town