> whether it was two pin compatible designs or one chip with both ISAs.

While AMD SkyBridge is going to implement the former idea, the latter idea may be tackled by another contender according to rumours:

http://www.extremetech.com/computing/185888-vias-new-isaiah-x86arm-hybrid-cpu-outperforms-intel-in-benchmarks-but-will-it-ever-come-to-market