There are some very old violin duet arrangements (dating back to Mozart's time - not sure whether he arranged them himself - maybe somebody else knows?) of arias from Don Giovanni and the Magic Flute. Some of them are not that easy, but Grade 8 standard means you should be able to give them a good go, and you probably know what they are supposed to sound like.
Also I can recommend the Telemann Canonic Sonatas. They are maybe grade 6 standard (ish), so you should find them very approachable, and they are really lovely.
Enjoy your duetting!