I have an investment joint with my mother. I would like to report the T5 without effecting my spouse.
To split your T5 with someone other than your spouse, simply report the portion that applies to you on your own tax return and provide the other joint account holder(s) with a copy of the T5 slip in order for them to report the proportionate income on their tax return.
Retain your copy of the tax slip and your explanation as to why you have not reported the full amount in your own file in case you are asked for it by CRA.
If you do not want to affect your spouse's income, you must only report "on your return" the amount of the T5 income that applies to you and leave the "percentage" field at 100%.