The votes will be held to replace two former ministers in the Liberal government.
One riding, Lafontaine, is a safe Liberal seat in east-end Montreal.
The other, Argenteuil, located between the Laurentians and the Ontario border, could serve as a better bellweather for Quebec's turbulent political climate, where three parties are in contention for power.
The Montreal seat was vacated by ex-family minister Tony Tomassi, who resigned amid ethics controversies. He now faces criminal charges related to alleged use of a credit card that belonged to a company that did business with the government.
David Whissell was a labour minister who resigned from that role amid conflict-of-interest allegations involving a family asphalt business that received government work. After two years as a backbencher, he left his Argenteuil seat to focus on his business career.
Premier Jean Charest must call a general election over the next year and a half. He has already said he won't call one for June.