Complex sentence: Lower Canada, Upper Canada and their legislatures were abolished by the British Act of Union 1840 of July 23,1840 (which came into force in early 1841) and united as one political entity, the Province of Canada.
Split into simpler sentences: Lower Canada, Upper Canada and their legislatures were abolished by the British Act of Union of July 23,1840, which came into force in early 1841. The act abolished the legislatures of both Lower and Upper Canada and united them as one political entity, the Province of Canada.