"
Can I link an application to sponsor's account or should I only link it to Primary Applicant's account?
You can link it to any account, whether it's PA's or sponsor's. There is no difference. Just remember, after you've linked an application, you will receive all the communication with CIC through this account. And if you've linked an application to sponsor's account, all notifications will come to sponsor's email.
Please also remember that you can just share credentials (a username and a password) with your partner and you both can check the same account.
Can I link an application to both sponsor's and PA's account at the same time?
No, you can't. It can only be linked to one account at a time. But if you've linked an application to one account and now you want to link it to another one, you still can do that. Just remember that once you link it to a new account, in the old account you will see a message that an application has been now linked to another account and you won't have access to it in the old account. In other words, after linking an application to a new account, an old account will lose a connection with it."
It is a link to a comprehensive threat dedicated to how to link and common questions
https://www.canadavisa.com/canada-immigration-discussion-board/threads/how-to-link-an-application-to-cic-account-step-by-step-instruction.619724/