I find concrete examples clearer to understand: John and Mary are married and contribute equally to an investment that produces a T5 form. The T5 is in Mary's name and box 13 reads 2000. When reporting the income, what value would John insert into Box 13 and what value would Mary insert into Box 13? Who puts the 50 into the "Enter percentage of slip to claim on your spouse's tax return (if applicable)"? Only Mary? Only John? Both?

Thanks in advance.

Mary reports the full amount of the T5 and Mary puts 50.0% in the box "Enter percentage of slip to claim on your spouse's tax return (if applicable)" John does not claim anything as the amount will automatically transfer to his spousal return in Schedule 4.

This technique for including joint ownership applies to the T3 and T5008 data as well as the T5.  While our investments are jointly owned the CRA autodownload feature only provides the T5, T3 and T5008 to one of us.  You do not need to reenter any of the information in the other owner's tax form, just enter your percentage and the balance is automatically added to the spouses tax forms.  (It took me 3 years to figure out how to do this but Turbotax was working correctly and automatically.)

Curious, can I split T5 income with my spouse even though the investment is only in my name?

@bobbyP No. The T5 needs to come from a joint account, not one that's just for one spouse only.

There is a big bug in turbotax. While preparing a couple tax downloaded T5 from cra site. It is from a joint account.

When I say 50% is spousal portion it puts 50% in spouse but it is still keeping 100% in my return.

So I am payiong tax on full income even though only 50% should be there. For the dividend income I checked the lines 12000 and 12100 in a test run. This is a big bug. Please fix it. I checked my returns for 2021 also it did the same mistake. I have to now correct and resubmit to cra explaining the problem. I am surprised that Turbo tax testers missed this. The onloy tested if the % given is transferred to spouse or not, but for got to reduce the amount used bu spouse in my return.

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report the investment income according to the attribution rules, not by the name on the slip. 

It is reported in proportion to who invested their money.  

"According to the CRA, interest earned on a joint account requires proportionate tax reporting, where each owner of a joint account reports their individual portion of the total interest. In other words, taxes are paid on the interest according to how much each co-holder contributed to the account."