 From discussion in https://github.com/quarto-dev/quarto-cli/issues/5349, a project having both file will be silent and it seems we can improve that. 
 
I think this is the function that detects both file 
https://github.com/quarto-dev/quarto-cli/blob/4a4e9f1226df1f6f994ac084b226150966bc7aa5/src/project/project-shared.ts#L90-L94

We could either warn or even error if we detect both            
cscheid
2023-05-03T15:06:07Z
cderv
enhancement triaged-to
open
Warn or error if both `_quarto.yml` and `_quarto.yaml` are found
https://github.com/quarto-dev/quarto-cli/issues/5397