As ever, it's not easy to make recommendations without knowing your voices - but this is a rich and rewarding field to explore.
You could look through the Purcell duets - 'Lost is my quiet' is particularly suited to lower voices - and, if you like baroque style, there are some beautiful Monteverdi ones. Moving on the the 19th century, there are volumes of Schumann, Mendelssohn and Brahms. Oh, and Dvorak ( Moravian Dances etc)
As for 'contemporary', my least favourite duet of all time is probably 'I know him so well'.. followed closely by 'It's her or me'.. (not that you asked

)
Good luck in finding something suitable.