For most people, one will is sufficient to properly protect their assets. Sometimes, however, it will be appropriate to have two or more wills in place to deal with different types of assets.
In Ontario, the concept of multiple wills has been recognized since the case of Granovsky Estate v. Ontario in 1998. Basically, one will is used for probatable assets (houses, bank accounts, investments, etc.) and one will is used for non-probatable assets, such as shares in privately-held companies. On the death of the testator, the assets in the second will are kept out of probate, and therefore kept free of probate taxes.
Multiple wills are not for every person or every situation, but they can be a very valuable tool in the right situation.