If both handsets have their ringer enabled, then yes, both will ring simultaneously with an incoming call.
Here are a few other points on this subject:
I have two handsets with my Panasonic phone. One is in my living room and the other is next to my bed. I don't want the phone in my bedroom to ring, so I have that ringer shut off. I can still hear the phone out in the living room ringing if someone calls in the middle of the night, but at least it's not a shock waking up to a ringing phone right next to my head.
The phone has a lot of other flexible options with the ringer that you can select. You can set the ringer volume higher or lower for each individual handset. You can also select a different ring tone for each handset.