Why a second OS though? Why not create a highly efficient co-processor that specialize in some tasks like security with TouchID and some other "helper" functions to OS X, much like Apples M series?
Several years ago i imagined a co-processor taking care of "useless" OS and kernal tasks like timers/timer interrupts, timer frequency etc, there is still some apps on OS X that really should be idling but still uses 1-2% cpu doing nothing other than ask to see if it have a task to do, wasting energy and CPU time for nothing, waking an idling CPU is wasting energy (on desktops its pretty much irrelevant but on laptop this could improve battery time), maybe pointless now since Maverick introduction of timer coalescing but improvements can obviously still be made.
But still, a custom chip that is super energy efficient that uses interrupt coalescing, timer coalescing, wake ups, or even is a controller/helper unit for the tickless kernel. There is some tasks that feels to be a waste of energy waking the big beast (Intel/AMD cpu:s) "Excuse me CPU, this is Microsoft Word, do you have a task for me or should i go back to sleep"
Thats just some examples, the ship could probably do alot more. Probably not that easy to implement a multi architecture like that though.