So, in case this question hasn't been asked, something I was curious to know. Can elemental resistances from multiple pieces of gear reduce damage further, and if so, how would it work (additive, multiplicative)?

I have a set of Diamond Armor from FF5 when I made a small Synergy Draw during Bartz. The Res on it isn't terrible, and it has a Lightning Resist. I was considering using it alongside a Resist accessory for the Gigas fight, since I know Thunder Gigas is going to take my party the longest to kill. Would throwing it on alongside a Bolt Ring or such reduce Thunder damage further?

Might also be worth knowing for when Kuja hits in a few events. With how absurd those numbers get, another 20% resistance might be worth the drop on the RES numbers.