If basing it off a "standard D&D setting," I wouldn't do it as a second Arcane Background.
The character already has the AB Edge, the effect is just gaining access to powers, activation, and trappings only allowed to another style under that Edge.
So it's a modification of an existing Edge, which is handled in Savage Worlds by a new and different Edge. I'd do it more like...
Dual Arcane Training
Requirements: Novice, Arcane Background (Magic, Miracles, or Psionics), Arcane Skills d6+
The character has access to a second type of Arcane Background from those listed in the requirements. For instance, if he has AB (Magic), he can choose Miracles or Psionics as his secondary choice.
He gains one power of the new type and activates it using the normal rules for that type of arcane power, but at a -1 penalty. He uses the same pool of Power Points (using one type of arcane power drains the ability to use the other equally), but powers for the secondary type cost +1 PP to activate as well.
Henceforth, when he takes the New Power Edge, he may choose to take a power from either arcane type, but must note which arcane activation applies.
Dual Arcane Mastery
Requirements: Veteran, Dual Arcane Training, Arcane Skills d10+
Powers for the secondary arcane type ignore the -1 penalty and no longer cost +1 PP to activate.