Each holder is responsible for their own %, if the account is held in both names, then you enter in their percentage while keeping the other account holder's % as well as SIN#; for your record, in case the CRA asks for it later.
It will be allocated according to how you've claimed the type of ownership of the rental property as a "co-ownership/ sole proprietor" instead of "partnership". If you change this selection to "partnership", this will be allocated according to the percentage of ownership entered.
Please see link below for further clarification:
I disposed of a rental property owned jointly with my husband. Yet, there is no option to show a 50/50 split in Schedule 3, and the whole Capital Gain is attributed to me. Same goes for stock disposition capital gain. Where can I find the split option?
Thank you in advance for your reply.