What merged canada's two parts into one single unit governed by a canadian legislature?

The answer is The Act of Union of 1840.

The Act of Union was passed by the British Parlament and proclaimed by the Crowd. It united the colonies of Upper Canada and Lower Canada under one government, creating the province of Canada.

The Act abolished the separates Parliaments of both colonies and replaced them with a single one with two houses. The Legislative Council was the upper chamber and the Legislative Assembly the lower.

The reasons for the unification were: the near bankruptcy of Upper Canada and an attempt to defeat the French vote by giving each of the former provinces the same number of parliamentary seats.