Ahmad Babba Kaita and Lawal Yahaya Gumau have been sworn in as senators representing Katsina north and Bauchi south respectively.
Both senators are of the All Progressives Congress (APC).
They were sworn in by Senate President Bukola Saraki at the upper legislative chamber on Wednesday.
Ahmad and Gumau had won the bye-elections conducted in August by a landslide.
Ahmad polled 224,607 votes ahead of Kabir Babba Kaita, his brother, who polled 59,724 votes. His brother is of the Peoples Democratic Party (PDP).
Gumau polled 119, 489 votes to beat Ladan Salihu of the PDP who got 50,256 votes.
The Independent National Electoral Commission (INEC) conducted by-elections in their areas following the demise of senators who had represented the senatorial districts.
Mustapha Bukar, representing Kastina north, died in April while Ali Wakili, representing Bauchi south, died in March.