Who would elect the senators

The Senate of the United States shall be composed of two Senators from each State, elected by the people thereof, for six years; and each Senator shall have one vote. The electors in each State shall have the qualifications requisite for electors of the most numerous branch of the State legislatures.

This expanded the franchise in some states. To balance this opening, the two Senators in the upper house of the national legislative branch would be elected by each state's Legislature.