Southcourt and Riverside wards will go to the polls in August after local councillors stepped down.
Cllrs Peter Agoro (Southcourt -Lib Dem) and Nick Lewis (Riverside - Conservative) cited personal reasons for their decisions to give up their seats.
The by-elections will take place on Thursday 17 August. Cllr Agoro has also resigned from the Southcourt Ward of Aylesbury Town Council so the polls will be combined.
There will also be a by-election for a vacancy on Slapton Parish Council on the 17 August (if contested).
The counting of votes for all contests will take place at the Gateway from 10.30pm on the same day.