Author separator "," vs "&"?
 

Does not seem to make difference weather i use coma "," as author separator or "&". However & is easier to see in book description.

Is it okay to use "&" as author separator?

Yes, because Calibre is designed only for use "&" as author separator.
The comma "," is the separator character for 'Name, First name' (often used invisibly for sorting for example).

'Frank Herbert & Brian Herbert' => 'Herbert, Frank & Herbert, Brian'

A text like 'Frank Herbert, Brian Herbert' will be invalid and does not allow normal use of Calibre.

Try it.
Calibre treats authors like "John Smith & Jan Newman" as a two separate authors and creates appropriate database entries for them.
Coma is usually used when you want to list names like "Smith, John" as some people prefer.