Ceremony music outside - duet or trio?

Up it at least a trio.

I’m a professional violinist and can say it honestly doesn’t matter, as far as volume is concerned. A good musician will be able to play at a sufficient level so the number of musicians won’t matter in that sense. However, if you want a fuller sound (meaning high and low pitched instruments) it would be best to have a violin/cello duo or violin/viola/cello or violin/violin/cello trio. You can never go wrong with string music- any combination of instruments you get will be beautiful!

If you need a place to cut your budget it'll be fine to go with just two. I second Heather's vote for including a cello either way!

Thanks for responding. Maybe I'm crazy but when I listened to music online to get an idea and it was just a cello and one violin. The cello was so overpowering sounding to me.

I would talk to them. Do you have a song in mind? They may be able to suggest arrangements based off your song choice.