Hi Kevin,
You CAN use both Wwise and Unity sound engine simultaneously but I personally think that using both Wwise and Unity's sound engine can be very cumbersome at the end of the development where you need to optimize audio since you will need to profile Wwise and Unity separately.
If you have someone in your team who specifically remembers all of the work done in both Wwise and Unity audio thoroughly and if that person stays in your team nearly forever, then it might be worth trying. I'm not being sarcastic, I'm pretty serious here because it can cause critical issues regarding optimization and division of roles between a sound designer and a programmer especially if they don't work with any one of either Wwise or Unity.
But again, that's my personal opinion based on my preference.
- Narie