Boxing & unboxing is the process of converting a primitive value into an object oriented wrapper class (boxing), or converting a value from an object oriented wrapper class back to the primitive value (unboxing).
The Old Dalaran will stay in Northrend, so yes, mages are going to have 2 portals to dalaran (one to Northrend and one to the Broken Isles)
Dalaran (aka Dalaran City) is a magocratic city-state which was once located by the Lordamere Lake on the Alterac Mountains shore in the Eastern Kingdoms.