I suspect the dual AES units are a side effect of Zen having two 128-bit SIMD units. The AES instructions use the SIMD registers so naturally the AES implementa

I suspect the dual AES units are a side effect of Zen having two 128-bit SIMD units. The AES instructions use the SIMD registers so naturally the AES implementation is integrated into the SIMD unit. Presumably it was easier to duplicate the AES engine along with the rest of the SIMD unit than to split it out.