Let me clarify this..
FBH adds +50% damage to demihuman, right? So it's a bonus2 bAddRace,RC_DemiHuman,50; effect.
Goldenring adds +25% damage to demihuman, so it's a bonus2 bAddRace,RC_DemiHuman,25; effect.
The game stacks all similar effects together before doing damage calculation, so with 2 FBHs and a goldenring it would total to bonus2 bAddRace,RC_DemiHuman,125; or +125% damage.
If you use a different effect, however, such as bonus2 bAddRace,RC_NonBoss,20; (turtle general), they don't get added together, since it's two different effects.
So with FBH FBH Goldering it sums up to (base damage * 225%), whereas for FBH FBH TG it sums up to (base damage * 200% * 120%), which equates to roughly (base damage * 240%).
tl;dr FBH FBH TG > FBH FBH Goldenring