Yea, if you only use water against fire, then the +10% fire resistance is really good. I'm not actually sure if double Jack Frosts stack, but to this point, eidolon passives have always stacked.
So let's say if you enter a fight against Fire with double Jack Frost; assuming they stack, that's 20% fire resistance for the entire fight. Not only is that -20% from all damage you'll be taking that fight, element resistance buffs stack additivelywith damage cuts and reflects. So if you use Joan of Arc's damage cut for example, that -40% gets added to the 20% fire resist so your damage_cut_total is 60% (ie, multiply damage by 0.4).