Hello,
These gears come in matched pairs. You may have wear issues just changing one gear. You may not!
I have seen just one changed and it worked fine. No guaranteed success unless both are changed. Also repair or replacement of the aluminum/bushing is most important or any combination will wear.
If you change just one gear, I would occassionally check the endplay on the vertical timing gear. This will indicate if you have any premature gear wear.
Besides being difficult to change, these gears are expensive!