Years ago it was easy to get a matched set belts that were of extremely close, if not identical length and sold as a "matched" set. When two belts are on the same pulley they will have different tension if they are not the
exact same length. If you can, go to a parts house that will let you measure and compare all of the belts they have in stock for the size needed, look at the lot numbers on the belts and get the two closest ones. You may also be able to measure them on a belt gauge at the store.
This is one of those small things that "makes the difference" and looking at that job you will see doing it once is too much
When installing the new belts line up the numbers and labels and it will be an indicator if one is slipping if the numbers no longer line-up.
Good luck!!!