I agree attribution based on contribution is the correct way, though I acknowledge CRA is unlikely to challenge a 50/50 split, particularly if that has been the reporting history of this account.Who contributed the money to the account that is creating the income? That is who must report it.
This is part of the CRA's attribution rules.
Added: Was looking for a link with a clear explanation. Perhaps this: https://turbotax.intuit.ca/tips/can-i-split-my-capital-gain-with-my-spouse-2-6360
And this: http://deanpaley.com/the-income-splitting-rules/