It took years grabbing these as they've popped up for decent prices, just recently grabbed V to finish it off. Wish III had been released as a GBA cart.
It was many moons ago, but I recall this is correct. You had to place locations in a specific pattern for some of the mission items and it was totally possible to do with a single cart, just you basically needed a guide to figure out all the combinations to create the right map. It was not a trivial thing, and I recall using gamefaqs a lot.
My understanding was you needed to have a second copy to 100% - I was always missing a specific mission item. So I never completed the game.
I recall there being a mission item that you need specific location placement to get so if you fucked up before hand, you couldn't get it.
It was many moons ago, but I recall this is correct. You had to place locations in a specific pattern for some of the mission items and it was totally possible to do with a single cart, just you basically needed a guide to figure out all the combinations to create the right map. It was not a trivial thing, and I recall using gamefaqs a lot.
Time to reboot and replay!!!