Many years ago I saw a brother and sister playing on the stage at the Kendallville, IN bluegrass festival. At one point the young lady was picking the guitar while fingering the fiddle she held and her brother was bowing the fiddle while fingering the guitar he held. I was in awe! Never even considered trying it myself cuz it just seemed so incredibly hard.
Wish I could remember their names but it was too long ago ..... way before youtube so I couldn't keep track of them after that show.
Here are a couple old pros who also knew that trick: youtu.be/9DoxwrSHKr0 Sometimes you see pictures of old stringbands sharing each other's instruments like that. For several years we played at the Old Dominion University folk festival in Norfolk, and one of the "highlights" was a mock "battle of the bands" with a local Bluegrass group, who were good sports. One year they came out with fake moustaches and cowboy hats and played one of our tunes. We decided to try that old trick and actually learned and performed "Soldier's Joy" that way - I don't remember the exact lineup, but everybody played half of two different instruments. Somebody probably took a picture, but fortunately I don't think any audio recordong exists!