Yes, you can split T5 investment income if it's from a joint account. If you are preparing a joint return, you would enter the slip on one spouse/partner's return and then enter their percentage of the investment. The rest will automatically go to the other spouse/partner.
You would report the split according to the attribution rules, in proportion to who invested their money. Ex: Mary reports the full amount of the T5 and Mary puts 25% in the box "Enter percentage of slip to claim on your spouse's tax return (if applicable)" because she only contributed 25% to the investment. John does not need to enter a T5 on his return. 75% of the amount of Mary's T5 will automatically transfer to his Schedule 4.
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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.)