The use of a single ballast for both tubes is much more common with the new narrow T5 light fittings (these lights have a narrow tube 16mm in diameter and are a bit shorter than the standard T8 (25mm diameter) fittings). So unfortunately in this case you won't be able to delamp and use reflectors. To be sure remove one tube and see if the other goes out.
One of our customers had a problem with their single T5 light fittings - they weren't producing enough light. By fitting reflectors the light output increased, and the indoor climbing plants the lights were illuminating started growing after the reflectors were fitted!