No, not unless they want to convert their own, singular interest into two or more shares. The reason for this is that grantors in a deed can only convey what they already own. If two tenant in common co-owners want to make themselves joint tenants, then they can collectively convey their interests to themselves in a deed. This works because, together, they own the whole property.
But if one co-owner conveys the property to himself, and declares that he is now a joint tenant, that doesn’t work. Only half the property is being conveyed, and only one person is receiving the property. This does not meet the requirements of California’s Civil Code, which requires that a joint tenancy vest in two or more persons with equal shares. (Civ. Code § 683.)