If I have two Gondorian Fire cards attached to my hero can I perform both actions once per phase? Or is Gondorian Fire itself limited to once per phase?

For instance, if my hero has 8 resources and 2 Gondorian Fire cards, can I spend two resources to get +12, or can I only spend one resource for +7?

I assume the former because the card itself is not limited to 1 per character.

Each copy of the card is limited to one trigger per phase, so in your situation you can use it twice total.

cf. http://www.lotr-lcg-quest-companion.gamersdungeon.net/#Rule145

Also, since lasting player card effects do not recalculate ( http://www.lotr-lcg-quest-companion.gamersdungeon.net/#Rule148 ), you actually get +13 attack in your stated situation, not +12.

Wow, cool. Thanks a lot.

As much as i personally hate this rule, sappidus is totally correct.

Up to you avoid it, if you feel it totally OP to the point to trivialize almost all boss fight....

The luxury of cooperation games

Would 2 uses be +13?

Yes, it's what Sappidus said.