Here is what I do, but in no means suggesting you do yourself (end disclaimer):
This is a popular method on the aquarium NGs, and is nothing new.
The Problem: I had older 4' 2-tube fixtures that flickered when the shop was chilly and were just plain dim on all other occasions. The ballasts were the old, heavy, inefficient magnetic ones that are so ubiquitous in shops. The fixtures themselves disassemble easily, and have plenty of room for the ballasts inside.
The Solution: Home Depot carries Advance Instant-Start Electronic Ballasts (REL-4P32-SC), This ballast is light as a feather, and designed for (3) or (4) bulbs, However; they will successfully operate (2) bulbs at double the line current, and double the brightness) if wired up so.
How to do: Remove the old ballast, toss in trash. Install the new ballast in place, it should be a direct replacement. Ignore the diagram on the label, take the two blue wires to one lamp socket, the two red wires to the adjacent socket. The two yellow wires go one each of the opposite sockets, with a short jumper wire connected between the two remaining terminals of those sockets. The ballast has thermal protection so will not explode if wired incorrectly. I cut, and splice using barrel connectors and crimp.
Get rid of those old tubes, and install some new T-8's. They are the 1" diameter tubes so popular in Canada, they are more efficient then the 1.5" tubes most of have. HD carries them too.
This works well for me, and thought I would share since I have taken so much helpful info from this NG.
Be careful,
DJ