Again, normally I keep my mouth shut around these parts, but what the hell, I'm feeling lucky today! Maybe also consider just getting a triangle of some percussion instrument to pick up while the singing is going on so that the dancers still have a beat to cut up to, but the continuance of the accordion or fiddle doesn't step on the singer. For instance, accordion starts the song with the fiddle playing a second or harmonizing melody. Then, when the singing kicks in, the accordion player picks up the triangle to continue the beat for the dancers. Lots of variation that you can do having a triangle or scrub-board (something like that) laying around for the opposite player to switch to at any point.
Hell, I'd also say consider letting the singer go a capella for a bit. With a nice enough voice and nice enough lyrics (this requirement is met by ALL Cajun songs), sometimes a capella can be VERY powerful to drive home the feel of the tune.
As I said, I normally keep quiet around these parts, but the holidays are doing something strange to me.
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.