Your will can transfer property to others in joint ownership (either in a tenancy in common, or a joint tenancy). You might want to consider such a bequest if you have a property that cannot be easily divided and there are two or more people whom you would want to benefit equally. Keep in mind that such an arrangement should only be considered if you're reasonably sure that these people can use and enjoy the property in harmony. If this is the case, joint ownership may be OK, but a tenancy in common might be more fair to the heirs of the beneficiaries, since the survivorship feature of a joint tenancy ensures that only the beneficiaries of the surviving joint tenant will ultimately get the property (unless the joint tenants agree to a partition of the property during their lifetimes).
You also might be tempted to use joint ownership as a means of influencing the behavior of one of your heirs. For instance, what if you would like one of your nephews to have a certain property, but aren't so sure that he is ready to handle it on his own. If you transfer it by will to the nephew in joint ownership with another heir — an older niece who you trust more — your nephew may be reined in somewhat by the other joint tenant.
While such a transfer might work as you want it to, there are many
problems that could occur. If you choose to transfer the properties to the heirs
in a joint tenancy, the "wrong" person might end up ultimately getting
the property (in the above example, the nephew might die first, in which case
his heirs get nothing). If a transfer into a tenancy in common is used, the
nephew might frustrate your attempt at control by selling his interest to a
third party. This is another situation where an attempt to influence the
behavior of other by way of a will provision in all likelihood could be handled
much better by a transfer into a trust.