Welcome. My name is XXXXX XXXXX would be glad to assist.
You have 2 options on the fixture. You can replace a single ballast or you can remove both of the existing and replace with a 4 lamp ballast
No problem, any concerns either way?
You will have to rewire the lamp ends when changing to the new electronic version if they are magnetic ballasts now
If it originally had two, its okay to replace with only one?
Absolutely, do it all the time. Usually if one goes out, the other one may not be far behind, so change to a 4 lamp and be good for another 10 years