My wife and I have such a duo, and we find that a really good sounding way to play is for both accordion and fiddle to play the melody together and then the fiddle seconds and I play the bass on the accordion while I sing. We have to be careful what chords we play, because I can’t play all of them. This is an old old way to play. This is the way Amede Ardoin and Dennis McGee played together. Too bad you can’t hear Dennis better on the records.
I forgot to mention that this is also how Hector Duhon and Octa Clark played. On that old album with the black and white cover, they have Michael Doucet playing guitar but you can tell they don't need him. Hector does some really interesting things playing around the melody, weaving in and out, but what he plays is mostly based on the melody.
We haven’t crossed that bridge yet because she hasn’t sung anything. I guess I would keep the rhythm going with the bass and use two or three note chords in the right hand, with some omissions where the bass and chord would clash.