Additionally, a creature can't attune to more than one copy of an
As both items are referred to with the same name (as opposed to the +2 being called Greater Rod of the Pact Keeper or such) this rule could be called into effect. However what constitues the "same" item is not clarified, so this is not strictly RAW.
So RAI points to a character being unable to attune to both.
If the GM decides that he considers these items different and allows attunement, both could be used once per long rest to regain a spell slot, just as any other pair of items could be used independently of each other.