No, you do not declare the social security benefits on both returns. The IRS Publication
915 as well as the one shown below confirm it. U.S. social security benefits paid to a resident of Canada are taxed in Canada as if they were benefits under the Canada Pension Plan, except that 15% of the amount of the benefit is exempt from Canadian tax.
You would not include your US Social Security benefits on a US federal tax return when those benefits are being reported on the Canadian tax return.
