I meant to ask if I could use one sensor to determine if it is dusk or dawn, and also use the same sensor to detect motion.
The MS14a will report motion at it's assigned address and dusk/dawn at the assigned address+1. You can't stop the dusk/dawn feature unless you disable/obscure the light sensor in the unit.
Are you trying to determine the amount of ambient light outside or in a particular room?
Either way, be very careful with the placement of your dusk/dawn sensor. It's difficult to find an ambient sensor location in your home that isn't affected by your interior lights. If it's outside, you'll need it in a location where it's not affected by your outside lights or headlamps.
I live in northern Indiana (I can throw a rock into Michigan) where we have some very dark days. I've tried various configurations with the MS14a (and other sensors) to measure ambient light and trigger lights. I wanted a system that would determine the ambient light in a "reference location" on a overcast day and enable my occupancy sensors. These configurations invariably "broke" when things were re-arranged in the house.
I have since reverted to simple scheduled events ( dusk - x hours for interior, dusk for exterior ) due to the lower maintenance.
I am not saying that an interior ambient light sensor can't be accomplished. If you have a stable (things don't move around) location shielded from interior lighting, you could tune the system to respond to dark days. In my house, this would require a "light box" where I could control the light entry and shield the sensor from interior lights. I actually contemplated this, but was overruled by the boss (She was correct - it would have been obtrusive).