In Coil, you could upgrade the sword and shield separately. The problem was you needed an extra upgrade item to do so (the accessory upgrade). This put a n extra burden on the pld if they needed both (extra mat) upgraded, or gave them an advantage if they only needed 1 (less tomes)

Requiring both makes then pay the same amount (for better or worse) as every other job to upgrade. H
They still have the advantage of being able to buy their sword a week earlier if they get the tome (which, honestly, no progression group will do, but still)