How do I enable income splitting? I'm filling with my spouse and we're in very different brackets, but our returns are the same together as done separately. Just T4s

Employment income (like what is shown on a T4) can't be split. Only pension income can be split, and even then, not all types of pension income are eligible to be split.

This CRA page has more info on pension splinting: https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/pension-income-splitting.html