Both seats were held by the ruling Samajwadi Party.
Jangipur seat in Ghazipur district fell vacant when sitting MLA and cabinet minister Kailash Yadav died in February after a brain haemorrhage, while bypoll in Bilari was necessitated following the death of 64-year-old Haji Irfan in a road accident on March 10.
Yadav, who held the Panchayati Raj portfolio in the Akhilesh Yadav cabinet, wielded considerable influence in eastern UP and his death was considered a setback for the SP.
Irfan, a first-time legislator, was killed when the vehicle in which he was travelling fell into a ditch.