Choosing between Violin 1 & 2 and Violin I & II

Does anyone know how to do this? I currently have a document where the violins use the I & II naming convention and the violas use 1 & 2. Where do you define which is used?

If you have both conventions running concurrently, it means one of them was overridden manually. Check the instrument names, to see if one of them was forced, instead of allowing the algorithm to number the instruments.

Or could the Violas be solo players and the Violins section players?

The setting you are looking for is in Engraving Options > Staff Labels.

Yes, it sounds like solo vs. section. I’ve run into this several times by accident. Sections are in Roman numerals, soloists are in Arabic numerals.