The ADT SmartThings security system is an ADT Alarm and Smartthings hub in one unit. You just need to need to make sure you buy the ADT Smartthings sensors if you want the sensor to report to ADT for monitoring. Any ADT Smartthing sensor can be used by Smartthings to do automatons.
There is no reason to run a ADT panel as main hub and a second Smartthings hub. The only reason I can think of someone may do that is if they already have a ton of stuff setup in SmartThings before getting the ADT SmartThings Panel, or there is some legacy functionality they are afraid to lose like something with Smart Home Monitor. There is a smartapp called ADT tools though that should help alleviate that as well.
Smartthings store with all ADT stuff.
Another thread I discussed this and went into detail about the ADT Smartthings panel