The feat you're thinking of is in Races of Destiny, and is a Half-Elf only feat. All it does is increase synergy bonuses by 1. Considering how few +1s that tends to end up being and the fact that playing a Half-Elf on its own is like nerfing yourself...I don't consider it a good feat.
I think it's called Complementary...Insight? It's in RoD, in any case.