The Canada Revenue Agency allows you to pool together the charitable donations you and your spouse make and claim them on one tax return if you so desire. This is beneficial since it would increase the tax savings for the household overall. For example, let's assume that a married couple John and Mary (living in Ontario) both make eligible charitable contributions of $200 to registered Canadian charities for the 2007 tax year. If they were to each claim the donations on their respective tax returns they would each generate a tax savings of $43.10 each for a total household savings of $86.20. If they were to instead claim the charitable donations on only one of their returns they would generate a total tax savings of $135.92. This is because the first $200 generates a non-refundable tax-credit equivalent to the lowest marginal tax bracket in your province and any amount over $200 generates a credit based on the highest marginal tax bracket in your province.
Let's do the math:
Scenario 1 - Each spouse donates $200 to charity and makes the claim on their respective returns
John claims $200 which earns a credit of $200 x 21.55% = $43.10 tax credit
Mary claims $200 which earns a credit of $200 x 21.55% = $43.10 tax credit
Total household credits earned = $43.10 + $43.10 = $86.20
Scenario 2 - Each spouse donates $200 to charity and they claim $400 on only one tax return
John claims all $400 on his return.
Therefore the first $200 earns a credit of 21.55% which equals: $200 x 21.55% = $43.10.
The second $200 earns a credit of 46.41% which equals: $200 x 46.41% = $92.82
Total household credits earned = $135.92
Total tax savings versus each spouse claiming $200 on their respective tax returns = $49.72
Summary
You want to avoid getting credited at the low rate for the first $200 twice by only claiming charitable donations on one spouse's return. Note that a tax credit reduces your tax owing dollar for dollar no matter your marginal tax bracket. So if you have earned $135.92 in tax credits as above, then you will reduce your taxes payable by that amount. Since the Charitable Donation Tax Credit is a non-refundable tax credit, you will want to make sure that the person's return you are claiming the credit on has a tax liability of at least that much - otherwise you will be wasting some of the credit needlessly.